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The Story Of Life: Biology 11
Transcript of The Story Of Life: Biology 11
The Story Of Life
A virus is not classified as a living thing because it is Non-cellular, don't make or use energy, don't grow, can't reproduce on their own, don't respond to their environment, and can be crystallized. all of which are non-living qualities. although viruses do have living attributes it also has non living things about it. There are four different shapes viruses come in.
Protists are Plant-like that produce a lot of the oxygen in earth’s atmosphere and account for over 70% of photosynthesis on earth. Diatoms accumulate over millions of years forming “diatomaceous earth” which can be used in polishes, toothpastes and filters and as a natural insecticide. The protists supply nutrients produced by photosynthesis and the corals and clams provide a home for the phytoplankton this is a form of mutualism.
A plant is a multicellular eukaryote. Most plants can produce their own feed in the form of glucose through the process of photosynthesis. Plant cells have thick cell walls made of cellulose. The stem and leaves of the plant have waxy water proof coating called a cuticle(meaning skin in latin) to keep the water in.
We use plants for just about everything. The air we breath, for food, clothing, for medical uses like medicine etc. without plants human life would not be possible. Plants can be your ally as long as you use them cautiously. The key to the safe use of plants is positive identification whether you use them as food or medicine or in constructing shelters or equipment.
Cnidarians are a group of invertebrates made up of more than 9000 species of jellyfishes, corals, sea anemones, and hydras. They can be found worldwide, and all but a few cnidarians live in marine biomes.
To most people, the word worm describes a spaghetti-shaped animal. Many animals have this general appearance, but are classified into different phyla. The least complex worms belong to the phylum Platyhelminthes. These flatworms are acoelomates with thin, solid bodies. They range in size from 1 mm up to several meters. There are approximately 14 500 species of flatworms found in marine and freshwater environments and in moist habitats on land.
Helical- a hollow cylinder in the shape of a coil.
Polyhedral-a regular shape of many sides usually 20 sides.
Enveloped- a helical or polyhedral particle surrounded by an envelope.
Complex-a wide variety of other irregular shapes
There are two main types of viral reproduction. Lytic where the virus attaches and it inserts its genetics into a host cell. there its genetic material takes over the cell's activities, and the cell makes parts of future viruses, new biruses assemble and the cell explodes and the new viral particles are released . In Lysogenic reproduction the virus attaches and injects the genetic material into the host cells DNA. Viral genes go dormant for an unknown time frame but are copied with the host cells DNA the cell will switch into re-entering the cycle at synthesis releasing many viruses.
A bacteria all are unicellular, have a cell wall made of peptidoglycan ( a polymer consisting of sugars and amino acids), a cell membrane and no membrane bound organelles. They have cytopasm, ribosomes, and a single circular chromosome.
they are able to reproduce Asexually by means of binary fission and Sexually by conjugation. to reproduce asexually it splits in two equal halves. first the chromosome duplicates then the cell grows and eventually divides each cell containing one chromosome and they are both identical. the whole process can happen in about 20 minutes.
A bacteria cell reproduces sexually also. two cells come side by side joined by a bridge called a pilus. genes are transferred from the male to the female and spliced into the female chromosome then the cells separate.
Bacteria are not all bad. some bacteria we use for the good like:Nitrogen-fixing bacteria live in the roots of plants and create fertile soil Some species of bacteria clean up oil spills and break down toxic chemicals Some species of bacteria make food (yogurt, cheese). but there are parasitic bacteria like bacteria that are pathogens. Tuberculosis, Diphtheria, typhoid, fever, black death, anthrax, leprosy, and tetanus are all caused by pathogenic bacteria.
The cyanobacteria were called "blue-green algae" because of their ecology and their resemblance to the algae. However, cyanobacteria are prokaryotes, without a nuclear membrane and membrane-bound organelles, though they do have internal membranes. The algae are eukaryotes, having both a nuclear membrane and membrane-bound organelles. Although the Cyanobacteria were not the first cells to evolve on the planet, the "self-fossilizing" activity of Cyanobacterial mats has left us with the oldest fossils. They are procaryotic, have a cell wall, and produce slimy sheath that they float and some slide along.they reproduce by fission, some form spores and none are known to reproduce sexually.
Phylum Cyanophyta (Cyanobacteria)
Schizomycophyta are procaryotic (lack organized nuclei and membrane bound organelles), unicellular or colonial, most have cell walls and slime coats, most are heterotrophic (saprophytes, parasites), some are photosynthetic (some are both hetero. and auto.), some are chemosynthetic, and many are nonmotile but some are mobile. they are decomposers (break down dead organisms into compounds that can be used by living organisms - recycling the chemicals of nature.) Because they are nitrogen-fixing bacteria, we use them for: forming cheese, making vinegar, making butter and buttermilk, retting flax, making sauerkraut, and tanning leather plus much more.
There are three basic shapes of bacterial cells:
(use the shapes to classify the organism)
1. Coccus (plural Cocci) - spherical-shaped bacteria usually grow in clusters
2. Bacillus (plural Bacilli) - rod-shaped bacteria
3. Spirillum (plural Spirilla) - spiral-shaped bacteria
Diseases caused by protists are very dangerous because they can't be stopped by antibiotics. Many protists are parasites that live in body cells. Protists cause illnesses like: Malaria, African sleeping sickness, Changas disease, Toxoplasmosis etc...
Characteristics of Protists:
Contains the most diverse organisms of all of the kingdoms.
No such thing as a “typical protist”.
Can come in any size and both unicellular or multicellular.
The only thing they all have in common is that all are eukaryotic.
They are mostly fresh water unicellular organisms, this group has characteristics of both plants and animals. Some specimens are photosynthetic and others dont have chloroplasts and are heterotrophic. Some can even switch back and forth from autotrophs to heterotrophs and back. Many have flagella and are motile They reproduce only asexually through nuclear mitosis and cell division, ommon in ponds and pools, especially when the water has been polluted by runoff from fields or lawns on which fertilizers have been used. There are approximately over 1,000 species of euglenoids.
Fungi are important in ecosystems as decomposers that break down dead organic matter, releasing inorganic nutrients. They are also parasites of plants and animals.
Fungi are eukaryotic and heterotrophic. They have cell walls made of chitin, and they store carbohydrates as glycogen. In most species, the only diploid cell is the zygote
Fungi and animals produce chitin, are heterotrophs, and use glycogen as a storage carbohydrate. Molecular data support the close relationship between fungi and animals
Hyphae are filamentous structures that make up a fungal body. A mycelium is a visible mass of hyphae. Spores are the reproductive structures of fungi, and in many species a fruiting body produces the spores. The five phyla are distinguished by the presence of and type of sexual structures. The basidiomycetes produce sexual spores on a club-shaped structure; the ascomycetes produce sexual spores in sacs; the glomeromycetes have large asexual spores and lack a sexual cycle; the zygomycetes produce thick-walled sexual zygospores; and the chytridiomycetes produce sexual and asexual spores with flagella.
Zygomycota is commonly known as mold. Zygomycetes occur in decaying organic matter in soil (and they often decay spoiled food as well). They reproduce asexually when haploid hyphae create spore sacs that release haploid spores. They reproduce sexually when two hyphae fuse, merging their nuclei to form a diploid zygospore. A zygospore is formed by the union of two haploid hyphae. Inside the zygospore, two nuclei fuse into a zygote. The zygote then undergoes meiosis and develops into a spore sac that releases haploid spores.
Ascomycetes that are important to humans include the fungi that cause Dutch elm disease, chestnut blight, athlete’s foot, and other diseases. Penicillium notatum produces antibiotics and other Penicillium species flavor cheeses. Morels and truffles are ascomycetes. Yeasts are used in baking, brewing, winemaking, and production of soy sauce. The drug cyclosporine comes from ascomycetes.
In asexual reproduction, haploid hyphae produce haploid conidia by mitosis. In sexual reproduction, two haploid hyphae of compatible mating types fuse, producing a dikaryotic hypha. The nuclei fuse, forming the diploid zygote, which immediately undergoes meiosis (and usually mitosis) to produce eight haploid ascospores.
Dikaryotic cells form when two hyphae fuse but the nuclei remain separate. Inside the fruiting body, the nuclei fuse to form a diploid zygote. Meiosis follows, and the haploid spores form within asci. An ascus comes to contain eight ascospores after a diploid nucleus within an ascus undergoes meiosis. The result is four haploid nuclei, which each immediately undergo mitosis to produce eight nuclei (that mature into eight ascospores).
Familiar basidiomycetes include mushrooms, toadstools, puffballs, stinkhorns, shelf fungi, and bird’s nest fungi. Basidiomycetes typically produce four sexual spores per club-shaped basidium, whereas ascomycetes typically produce eight sexual spores in each saclike ascus.
Haploid basidiospores germinate, producing haploid hyphae. Two compatible hyphae fuse and form a dikaryotic mycelium, which grows within the food source. When conditions are right a mushroom is produced. Lining the gills are the basidia, where the two nuclei fuse to form the diploid zygote. The zygote undergoes meiosis, producing four haploid basidiospores. A “fairy ring” forms as hyphae grow outward from a food source, producing mushrooms near the outer edge of the ring.
Humans actually eat basidiomycetes! but there are toxic mushrooms in the world. Humans can eat dangerous basidiomyectes as a hallucinogen. most of the fungi in the picture behind are basidiomycetes.
Red allgae, members of this phylum aremostly multicellular marine seaweeds. the body of a seaweed is called athallus and lacks roots, stems, or leaves. Red algae use structures called Holdfasts to attach to rocks. They grow in tropical waters or along rocky coasts in cold water.
Red allgae has chlorohyll and photosynthetic pigments called phycobilins. Theses pigments absorb green, purple, and blue light (the only part of the light spectrum that penetrates water below 100m deep.) this means that red allgae can live in deep waters where other seaweed can not.
There are about 1500 species of brown allgae. Almost all of them live in salt water along rocky coasts in cool areas of the world. Brown algae contains Chlorophyll as well as a yellow brown carotenoid called fucoxathin, which gives them their brown colour. Many species of brown algae have air bladders that keep their bodies floating near the surface where light is available.
The largest and most complex of brown algae are kelp. Kelp has a thallus divided into the holdfast, stipe, and blade.
The Chlorophyta or green algae phylum consists of about 7,000 species, most live in fresh water, although some others are marine. Most green algae are microscopic, but a few species, such as those in the genus Cladophora, are multicellular and macroscopic. The cell walls of green algae are mostly constructed of cellulose, with some incorporation of hemicellulose, and calcium carbonate in some species. The food reserves of green algae are starch, and their cells can have two or more organelles known as flagella, which are used in a whip like fashion for locomotion. The photosynthetic pigments of green algae are chlorophylls a and b, and their accessory pigments are carotenoids and xanthophylls.
Some common examples of green algae include the unicellular genera Chlamydomonas and Chlorella, which have species dispersed in a wide range of habitats. More complex green algae include Gonium, which forms small, spherical colonies of four to 32 cells, and Volvox, which forms much larger, hollow-spherical colonies consisting of tens of thousands of cells.
Kingdom Plantae: Non-Vascular
Hepaticophytes include small plants commonly called liverworts. Their flattened bodies resemble the lobes of animals liver. Liverworts are non-vascular plants that grown only in moist places. water and nutrients move throughout a liverwort by osmosis and diffusion. Studies comparing the biochemmistry of different plants divisions suggest that liverworts may be the ancestors of all plants.
In non-vascular plants like mosses, the phase in the lifecycle called the "sporophyte" features structures that produce and release spores. The spores are essentially reproductive packets that are released into the environment. Since non-vascular plants are also non-flowering, and as such do not bear seeds, spores are technically not seeds.
The sporophyte may start out green but when mature is generally a brownish, somewhat wiry stalk with a capsule at its end containing the spores. It grows from the green, mossy part of the plant.
Under favorable conditions---moisture often being the most important condition---spores sprout and develop. Spores grow into either male or female individual plants. "Gamete" is the term for reproductive cells---sperm and egg---and "phyte" means plant. So "gametophyte" refers to the individuals in the stage of the moss plant's life cycle that bear the sperm and the egg.
The gametophyte is simply the familiar, "mossy" green part of the moss plant that we easily recognize.
People are often surprised to learn that plants produce eggs and sperm. In the moss plant, when a sufficient amount of water is present the sperm swim from the male gametophyte to the female.
Just as the sperm develop near the top of the green, male gametophyte of the moss plant, the egg develops near the top of the green, female gametophyte. The sperm eventually reaches the egg and fertilizes it.
Although it may sound like a science fiction term for a race of beings from another planet, the term "zygote" is a scientifically legitimate one. It simply refers to the fertilized egg. In the moss plant, the zygote forms near the top of the green, mossy female gametophyte. And from it grows a new sporophyte, complete with new spores, thereby completing the non-vascular plant life cycle.
Anthocerophytes are the smallest dicision of nonvascularplants, currently consisting of only 100 species. They are similar to liverworts in several ways. Likesome liverworts, hornworts have a thallose body. The sporophytes of these plants which resemble the horns of an animal, Give the plants their name Hornwarts. These non-vascular plants grow in damp shady habitats and rely on osmosis and diffusion to transport nutrients.
There are several divisions nonvascular plants, The first division is mosses or Bryophytes. They are the most familiar of the nonvascular plant division.Mosses are small plants with leafy stems. The leaves of mosses are usually one cell thick. Mosses have rhizoids, colourless multicellular structures, which help the stem stay attached to the soil. Although mosses do not contain true vasular tissue, some species do have a few long water conduction cells in their stems.
Mosses usually grow in dense carpets of hundresds of plants. Somehave upright stems others have creeping stems that hang from steep banks or rees. Some mosses form extensive mats that help retard erosion on exposed rocky slopes. A well known moss is Sphagnum (peat moss) this plant thrives in acidic bogs in northern regions of the world. it is harvested for and used as fuel. dried peat moss absorbs large amounts of water so florists and gardeners use it to increase the water holding ability of some soils.
Kingdom Plantae: Phylum Tracheophyta
Club mosses are vascular plants that live in moist environments. Lycophytes have stems, roots, and leaves. Their leaves contain vascular tissues even though they are very small. Species existing today are usually less than 25 cm high, but their ancestors grew as tall as 30 m and provided a large part of the vegetation of the Paleozoic forests. The plants ofthese ancient forests have become a part of the coal that is now used by people of fuel.
Horsetails represent a second group of ancient vascular plants. Horsetails were tree sixed members of the forest community. Now a days they grow as tall as 1m tall, there are about 15 species in existence. they get their name from the bushy apearance of some species. These plants also are called scouring rushes because they contain silica which we use for cooking. Most horse tails are found in marshes , shallow ponds or any area with damp soil. some species are common in drier soil of fields and road sides. The stem of the horsetail is ribbed and hollow and looks jointed. at each "joint" there is a cuff of tiny leaves. Sphenophyta are monocot.
Conifers grow in a wide range of habitats. Depending on the species, conifers can be tall or ground shrubs. Pine , spruce, juniper, red wood, fir, yew, hemlock arborvitae, cedar etc.. these are all conifers, cone bearing trees suck as pine, fir, cypress, and redwood.Conifers are vascular seed plants that produce seeds in cones. Species of conifers can be identified by the characteristics of their cones or leaves that are needle like or scaly. Bristlecone pines, the oldest known living trees in the world, a remembers of this plant division.Another type of conifer, the pacific yew is a source of cancer- fighting drugs
Gymnosperm means "naked seed" and is used with these plants because their seeds are not protected by a fruit. The gymonsperm plant divisions include Cycadophyta Ginkgophyta, Gnetophyta, and coniferophyta. Most conifers are monoecious which means they have both male and female reproductive parts on the same plant, most cycads, Ginkgoes are diocious, which means that male and female reproductive parts are on different plants.
Gymnosperms usually have two different types of cones: male and female. The male cones have pollen, and the female cones are essentially the "seeds." The pollen gets to the female parts by the wind, or pollinators.
Pterophyta are the most well known non-seed vascular plants. Pterophyta, commonly known as ferns, were abundant in paleozoic and mesozoic forests. They have leaves called fronds that vary in length. The large sixe and complexity of fronds is one difference between pterophytes and other groups of seedless vascular plants. Although ferns are found nearly everywhere, most grow in the tropics
Phylum Chrysophyta - Diatoms and Golden Algae diatoms are photosynthetic, unicellular organisms double shells of silica - Resemble box with lid use chlorophylls a and c, and carotenoids fossilize well - thick sediments of fossil diatoms are called “diatomaceous earth” Some move by secretions from shell Asexual reproduction separates shell halves each half produces new shell within old one - become smaller with each division Have gametic meiosis - cells are diploid and produce sperm or eggs by meiosis Golden Algae - use yellow and brown carotenoid pigments, and xanthophyll accessory pigments Unicellular, two flagella, often colonial, common in freshwater Form resistant cysts when ponds dry out in summer
Dinoflagellates are unicellular, photosynthetic, mostly marine, some bioluminescent Distinctive flagella and coat two flagella beat in grooves coat composed of cellulose plates Most use chlorophyll a & c and carotenoids Some are symbiotic with animals sea anemones, zollusks and corals in corals - called “zooxanthellae”, required for formation of coral reefs some forms cause “red tide” large blooms result in red colored water release toxins that kill fish and shellfish consumption of poisoned fish can kill Reproduce primarily asexually by longitudinal cell division with nuclear mitosis like fungi
Zoomastigotes are Unicellular, heterotrophic, highly variable in form Possess one to thousands of flagella Some free-living, some parasitic Some reproduce asexually only One group alternates between amoeboid and flagellated stages Some trypanosomes are human pathogens cause sleeping sickness, East Coast fever, chagas' disease many spread by insects, such as tsetse flies Some inhabit guts of wood-eating insects have enzymes capable of digesting cellulose Choanoflagellates are similar to feeding cells of sponges and are likely ancestors of all animals Hiker's Diarrhea: Caused by Giardia lamblia, found world-wide occurs in water, infects wild and domesticated animals, and humans Lives in small intestine of host Spreads as cysts in feces, can survive for months in cool water May appear in city water supplies Resistant to treatment with chlorine and iodine, requires boiling water to kill.
Phylum Ciliophora - The Ciliates
Unicellular, heterotrophic, with many cilia Coordinated beating provides motility outer pellicle is tough but flexible Two types of nuclei micronuclei - diploid - reserved for sex macronuclei - polyploid - for normal cellular metabolism Specialized vacuoles ingest food and contractile vacuoles regulate water balance Food enters through gullet (cytostome) and passes into vacuoles where it is digested Asexual reproduction by transverse fission Sexual reproduction by conjugation Two different mating types exchange haploid micronuclei Macronucleus in each individual disintegrates Multiple rounds of chromosomal replications in micronuclei reconstitutes macronucleus
Phylum Apicomplexa - Sporozoans
Nonmotile, spore-forming animal parasites Have an “apical complex” at one end of cell - with fibrils, microtubules, and vacuoles - used to enter host cells Have complex life cycles with sexual and asexual phases Exhibit alternation of haploid and diploid generations Fusion of gametes produces a thick-walled cyst, the oocyst Meiotic divisions in oocyst produce infective haploid spores, sporozoites Plasmodium causes malaria Gametocytes become gametes in gut of mosquito syngamy forms zygote and oocyst meiosis in oocyst forms sporozoites Malaria
estimated that 500 million infected, 200 million humans die each year, most infected children die symptoms include chills, fever, sweating, enlarged spleen, confusion, thirst - repeating every 48 to 72 hours Victims die of anemia, kidney failure, brain damage Effects can be reduced with drugs Focus is on eradication of malaria through elimination of mosquito carriers Vaccines against malaria may be available in near future.
Phylum Myxomycota -Plasmodial Slime Molds
Consist of streaming multinucleate plasmodiumfeeding phase may be yellow, orange or other color Cytoplasm exhibits conspicuous streaming Engulf and digest bacteria, yeast, bits of organic matter forms sporangium under adverse conditions Phylum Oomycota - water molds, rusts, mildew Live in freshwater or soil, many are plant or animal parasites Cell walls are composed of cellulose or similar polymers Body consists of filamentous hyphae Hyphae are diploid (unlike fungi) and produce gametes by meiosis Exhibit normal mitosis (unlike fungi) have unique life cycle Diploid spores produced asexually in sporangiumSex: female gametangium called oogonium with one to eight eggs male gametangium called antheridium with many sperm Fusion produces zygote that becomes thick-walled oospore. oospore germinates and forms new hypha
Psilophyta, Also known as whisk ferns have thin green stems. the psilophytes are unique vascular plants because they have neither roots or leaves. small scales that are flat, rigid, overlapping structure cover each stem. The two known generations of whisk ferns are tropical or subtropical. Only one genus is found in the southern US.
Kingdom Plantae: Angiosperms
Angiosperm means covered seed. flowering plants are classified in the division Anthophyta. They are the most well known plants on earth with more than 250, 000 known species. They have roots, stems, and leaves. but unlike other seed plants anthophytes produce flowers and form seeds inside fruit. Many different species of flowering plants inhabit tropical forests.
Monocots and Dicots
Anthophyta is unique because it is the only division in which plants have flowers and produce fruits. a fruit develops from flower's female reproductive structures. Sometimes other flower parts becuase part of the fruit and the fruit develops from more than one flower (pineapple). an advantage of fruit enclosed in seeds is the protection the fruit gives for the embryo, and often its an aid in the dispersal of seeds. Animals may eat them or carry them off to a different location (squirrels).
all animals have several characteristics in common. Animals are eukaryotic, multicellular organisms with ways of moving that help them reproduce, obtain food and protect themselves. Most animals have specialized cells that form tissues and organs. Such as nerves and muscles. Unlike plants, animals are composed of cells that don't have cell walls.
One characteristic common to all animals is that they are heterotrophic (they must consume food to obtain energy and nutrient. All animals depend directly or indirectly on autotrophs for food.
We believe that animals first evolved in water. Water is denser and contains less oxygen than air but water usually contains more food. In water some animals ,such as barnacles and oysters, don't move from place to place and have adaptations that allow them to capture food from their water environment. Some animals such as coral and sponges move about only during early stages of their lives. They hatch from fertilized eggs in to free swimming larva forms. Most adults are sessile and attach themselves to rocks or other things.
Sponges are asymmetrical aquatic animals that have a variety of colours, shapes, and sizes. Many are bright shades of red orange and yellow. Some sponges are ball shaped others have many branches. Sponges can be smaller than a quarter or as large as a door. Although sponges do not resemble more familiar animals, they carry on the same life processes as all animals.
Sponges are classified in the invertebrate phylum Porifera, which means “pore bearer.” More than 5000 species of sponges have been described. Most live in marine biomes, but about 150 species can be found in freshwater environments. Sponges are mainly sessile organisms. Because most adult sponges can’t travel in search of food, they get their food by a process called filter feed- ing. Filter feeding is a method in which an organism feeds by filtering small particles of food from water that pass by or through some part of the organism. How does a sponge get rid of its wastes?
Sponges have no tissues, organs, or organ systems. The body plan of a sponge is simple, being made up of only two layers of cells with no body cavity. Between these two layers is a jellylike substance that contains other cells as well as the components of the sponge’s internal support system. Sponges have specialized cells that perform all the functions necessary to keep them alive.
Sponges can reproduce asexually and sexually. Depending on the species, asexual reproduction can be by budding, fragmentation, or the formation of gemmules. An external growth, called a bud, can form on a sponge. If a bud drops off, it can float away, settle, and grow into a sponge. Sometimes, buds do not break off. When this occurs, a colony of sponges forms. Often, fragments of a sponge break off and grow into new sponges.Some freshwater sponges produce seedlike particles, called gemmules, in the fall when waters cool. The adult sponges die over the winter, but the gemmules survive and grow into new sponges in the spring when waters warm.
Sponges are soft-bodied invertebrates, that can be found at depths to about 8500 m. Their internal structure gives them support and can help protect them from predators. Some sponges have sharp, hard spicules located between the cell layers. Spicules may be made of glasslike material or of calcium carbonate. Some species, such as the lake sponges have thousands of tiny, sharp, needlelike spicules that make them hard for animals to eat. Other sponges have an internal frame- work made of silica or of spongin, a fibrous protein- like material. Sponges can be classi- fied according to the shape and makeup of their spicules and/or frameworks. Besides sharp spicules, some sponges may have other methods of defense. Some sponges contain chemicals that are toxic to fishes and to other predators. Scientists are studying sponge toxins to identify those that possibly could be used as medicines.
Cnidarians are a group of organisms but all have the same basic body structure A cnidarian’s body is radially symmetrical. It has one body opening and is made up of two layers of cells. The protective outer layer of cells develops from the ectoderm of the cnidarian embryo. The endoderm of the cnidarian embryo evelops into the inner layer of cells. The two cell layers are organized into tissues with specific functions. For example, the inner layer is adapted ainly to assist in digestion.
All cnidarians have the ability to
reproduce sexually and asexually.
Sexual reproduction occurs in only
one phase of the life cycle. It usually
occurs in the medusa stage, unless
there is no medusa stage then the
polyp can reproduce sexually.
Because a cnidarian’s body is only two layers of cells, no cell is ever far from water. Oxygen dissolved in water can diffuse directly into body cells. Carbon dioxide and other wastes can move out of a cnidarian’s body cells directly into the surrounding water.
There are four classes of cnidarians: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa. Cubozoans once were clas- sified as scyphozoans. The class Hydrozoa includes two groups—the hydroids, such as hydra, and the siphonophores, including the Portuguese man-of-war. Most hydroids are marine animals that consist of branching polyp colonies formed by budding, and are found attached to pilings, shells, and other surfaces. The siphonophores include floating colonies that drift about on the ocean’s surface. Hydrozoans have open gastrovascular cavities with no internal divisions.
It’s difficult to understand how the Jellyfish could be a closely associated group of individual animals. The Portuguese man-of-war, Physalia, is an example of a siphonophore hydrozoan colony. Each individual in a Physalia colony has a function that helps the entire organism survive. For example, just one individual forms a large, blue, gas-filled float. Regulation of gases in the float allows the colony to sink to lower depths or rise to the water’s sur- face. Other polyps hanging from the float have functions, such as reproduction and feeding. The polyps all function together for the survival of the colony.
The fragile and sometimes luminescent bodies of jellyfishes can be beautiful. Some jellyfishes are transparent, but others are pink, blue, or orange. The medusa form is the dominant stage in this class. The gastrovascular cavity of scyphozoans has four internal divisions. Like other cnidarians, scyphozoans have musclelike cells in their outer cell layer that can contract. When these cells contract together, the bell contracts, which propels the animal through the water. Jellyfishes can be found everywhere in the oceans, from arctic to tropical waters. They have been seen at depths of more than 3000 m. Swimmers should avoid jellyfishes because of their painful stings.
Anthozoans are cnidarians that exhibit only the polyp form. All anthozoans have many incomplete divisions in their gastrovascular cavities. Sea anemones are anthozoans that live as individual animals, and are thought to live for centuries. They can be found in tropical, temperate, and arctic seas. Some tropical sea anemones may have a diameter of more than a meter. Corals are anthozoans that live in colonies of polyps in warm ocean waters around the world. They secrete protective, cuplike calcium carbonate shelters around their soft bodies. Colonies of many coral species build the beautiful coral reefs that provide food and shelter for many other marine species. Corals that form reefs are known as hard corals. Other corals are known as soft corals because they do not build such structures. When a coral polyp dies, its shelter is left behind, which adds to the coral reef’s structure.
The most well-known members of this phylum are the parasitic tapeworms and flukes, which cause diseases in other animals, among them frogs and humans. The most commonly studied flatworms in biology classes are the free-living planarians.
Like many of the organisms in this phylum, most flatworms including planarians, are hermaphrodites. During sexual reproduction, individual planarians exchange sperm,which travel along special tubes to reach the eggs. Fertilization occurs internally. The zygotes are released in capsules into the water, where they hatch into tiny planarians.
Planarians also can reproduce asexually. When a planarian is damaged, it has the ability to regenerate, or regrow, new body parts. Regeneration is the replacement or regrowth of missing body arts. issing body parts are replaced through cell divisions. If a planarian is cut horizontally, the section containing the head will grow a new tail, and the tail section will grow a new head. Thus, a planarian that is damaged or cut into two pieces may grow into two new organisms.
The body of a tapeworm is made up of a knob shaped head called a scolex, and detachable, individual sections called proglottids. A proglottid contains muscles, nerves, flame cells, and male and female reproductive organs. Each proglottid can contain up to 100 000 eggs. Some adult tapeworms that live in animal intestines can be more than 10 m in length and consist of 2000 proglottids.
fluke is a parasitic flatworm that spends part of its life in the internal organs of a vertebrate, such as a human or a sheep. It obtains its nutrition by feeding on cells, blood, and other fluids of the host organism. Flukes have complex life cycles that can include one, two, or more invertebrate and/or vertebrate hosts.
Blood flukes of the genus Schistosoma, as shown in Figure 26.18, cause a disease in humans known as schisto- somiasis. Schistosomiasis is common in countries where rice is grown. Farmers must work in standing water in rice fields during planting and harvesting. Blood flukes are common where the secondary host, snails, also are found.
Roundworms belong to the phylum Nematoda. They are widely distributed, living in soil, animals, and both freshwater and marine environments. More than 12 000 species of roundworms are known to scientists. Most roundworm species are free-living, but many are parasitic. Nearly all plant and animal species are affected by parasitic roundworms. Roundworms are tapered at both ends. They have a thick outer covering, which they shed four times as they grow, that protects them in harsh environments. Roundworms look like tiny, wriggling bits of thread. They lack circular muscles but have lengthwise muscles. As one muscle con- tracts, another muscle relaxes. This alternating contraction and relaxation of muscles causes roundworms to move in a thrashing fashion. Roundworms have a pseudocoelom and are the simplest animals with a tubelike digestive system. Unlike flatworms, oundworms have two body openings—a mouth and an anus. The free-living species have well- developed sense organs, such as eyespots, although these are educed in parasitic forms.
Infection by Ascais is the most common roundworm infection in humans. It occurs worldwide but is more common in subtropical or tropical areas. Children become infected more often than adults do. Eggs of Ascaris are found in soil and enter a human’s body through the mouth. The eggs hatch in the intestines, move into the bloodstream, and eventually to the lungs, where they are coughed up, swal- lowed, and begin the cycle again. Pinworms are the most common human roundworm parasites in the United States. The highest incidence of infection is in children. Pinworms are highly contagious because eggs can survive for up to two weeks on surfaces. Its life cycle begins when live eggs are ingested. They mature in the host’s intestinal tract. Female pin- worms exit the host’s anus—usually as the host sleeps—and lay eggs on nearby skin. These eggs fall onto bedding or other surfaces. Trichinella causes a disease called trichinosis roundworm can be ingested in raw or undercooked pork, pork products,or wild game. Trichinosis can be con trolled by properly cooking meat. Hookworm infections are common in humans in warm climates where they walk on contaminated soil in bare feet. Hookworms cause people to feel weak and tired due to blood loss.
Arthropods pollinate many of the flowering plants on Earth. Some arthropods spread plant and animal diseases. Despite the enormous diversity of arthropods, they all share some common haracteristics. A typical arthropod is a segmented, coelomate invertebrate animal with bilateral symmetry, an exoskeleton, and jointed structures alled appendages. An appendage is any structure, such as a leg or an antenna, that grows out of the body of an animal. In arthropods, appendages are adapted for a variety of purposes including sensing, walking, feeding, and mating.Arthropods are the earliest known invertebrates to exhibit jointed appendages. Joints are advantageous because they allow more flexibility, especially in animals that have hard, rigid exoskeletons. Joints also allow powerful movements of appendages, and enable an appendage to be used in many different ways. For example, the second pair of appendages is used for mating in spiders and for seizing prey in scorpions.
Most arthropod species have separate males and females and reproduce sexually. Fertilization is usually internal in land species but is often external in aquatic species. A few species, such as barnacles, are hermaphrodites, animals with both male and female reproductive organs. Some species, including bees, ants, aphids, and wasps, exhibit parthenogenesis, a form of asexual reproduction in which a new individual develops from an unfertilized egg.
Diplopoda and Chilopoda
Centipedes, which belong to the class Chilopoda, and millipedes, members of the class Diplopoda. Like spiders, millipedes and centipedes have Malpighian tubules for excreting wastes. In contrast to spiders, centipedes and millipedes have tracheal tubes rather than book lungs for gas exchange. Centipedes are carnivorous and eat soil arthropods, snails, slugs, and worms. The bites of some centipedes are painful to humans. A millipede eats mostly plants and dead material on damp forest floors. Millipedes do not bite, but they can spray foul-smelling fluids from their defensive stink glands. You may have seen their cylindrical bodies walking with a slow, graceful motion.
Crustaceans (krus TAY shuns) are the only arthropods that have two pairs of antennae for sensing. Some crustaceans have three body sec- tions, and others have only two. All crustaceans have mandibles for crushing food and typically have two compound eyes, often located on movable stalks. Unlike the up-and- down movement of your jaws, crus- tacean mandibles open and close from side to side. Many crustaceans have five pairs of walking legs. A crustacean’s walking legs are used for walking, seizing prey, and clean- ing other appendages. The first pair of walking legs are often modified into trong claws for defense.Members of the class Crustacea include crabs, lobsters, shrimps, cray- fishes, water fleas, pill bugs, and barna- cles, shown in Figure 28.10. Most crustaceans are aquatic and exchange gases as water flows over feathery gills. Sow bugs and pill bugs, two of the few land crustaceans, must live where there is moisture, which aids in gas ex- change. They are frequently found in damp areas around building founda- tions. You can observe crustaceans in the BioLab at the end of this chapter.
Out of 35,000 species of spiders in the world, only about a hand full or two are dangerous to humans. Spiders, scorpions, mites, and ticks belong to the lass Arachnida (uh RAK nud uh). Spiders are the largest group of arachnids. Spiders and other arachnids have only two body regions—the ephalothorax and the abdomen. Arachnids have six pairs of jointed appendages. The first pair of appendages, called chelicerae (chih LIH suh ree), is located near the mouth. Chelicerae are often modified into pincers or fangs. Pincers are used to hold food, and fangs inject prey with poison. Spiders have no mandibles for chew- ing. Using a process of extracellular digestion, digestive enzymes from the spider’s mouth liquefy the internal organs of the captured prey. The spi- der then sucks up the liquefied food. The second pair of appendages, called the pedipalps (PE dih palpz), are adapted for handling food and for sensing. In male spiders, pedipalps arefurther modified to carry sperm dur ing reproduction. The four remaining appendages in arachnids are adapted as legs for locomotion. Arachnids have no antennae. Most people know spiders for their ability to make elaborate webs. Although all spiders spin silk, not all make webs. Spider silk is secreted by silk glands in the abdomen. As silk is secreted, it is spun into thread by structures called spinnerets, located at the rear of the spider. How does a pider catch its prey? Find out inFigure 28.9 on the opposite page.
Arachnida also includes ticks, mites, and scorpions. Ticks and mites differ from spiders in that they have only one body section, as shown in Figure 28.8. The head, thorax, and abdomen arecompletely fused. Ticks feed on blood from reptiles, birds, and mammals. They are small but capable of expand- ing up to 3 times or more of their orig- inal size after a blood meal. Ticks also can spread diseases. Mites feed on fungi, plants, and animals. They are so small that they often are not visible to the unaided human eye. However, you can cer-tainly feel their irritating bites. Liketicks, mites can transmit diseases. Scorpions are easily recognized by their many abdominal body seg- ments and enlarged pincers. They have a long tail with a venomous stinger at the tip. Scorpions live in warm, dry climates and eat insects and spiders. They use the poison in their stingers to paralyze large prey organisms.
Have you ever launched an ambush on a fly with your rolled-up news- paper? You swat with great accuracy and speed, yet your prey is now firmly attached upside down on the kitchen ceiling. How does a fly do this? The fly approaches the ceiling right- side up at a steep angle. Just before impact, it reaches up with its front legs. The forelegs grip the ceiling with tiny claws and sticky hairs, while the other legs swing up into position. The flight mechanism shuts off, and the fly is safely out of swatting distance. Adap- tations that enable flies to land on ceil- ings are among the many that make insects the most successful arthropod group. How is the ability to fly an adaptive advantage to insects? Find out in Figure 28.13 on the next page. Flies, grasshoppers, lice, butterflies, bees, and beetles are just a few mem- bers of the class Insecta, by far the largest group of arthropods. Insects have three body segments and six legs. There are more species of insects than all other classes of animals combined. You can find out more about insects on pages 1080–1083 in the Focus On.
Insects that undergo metamorphosis usually go through four stages on their way to adulthood: egg, larva, pupa, and adult. The larva is the free-living, wormlike stage of an insect, often called a caterpillar. As the larva eats and grows, it molts several times. The pupa stage of insects is a period of reorganization in which the tissues and organs of the larva are broken down and replaced by adult tissues. Usually the insect does not move or feed during the pupa stage. After a period of time, a fully formed adult emerges from the pupa. The series of changes that occur as an insect goes through the egg, larva, pupa, and adult stages is known as complete metamorphosis. In winged insects that undergo complete metamorphosis, the wings do not appear until the adult stage. More than 90 percent of insects undergo complete metamorphosis. Other insects that undergo complete metamorphosis include ants, beetles, flies, and wasps. Complete metamorphosis is an advantage for arthropods because larvae do not compete with adults for the same food. For example, butterfly larvae (caterpillars) feed on leaves, but adult butterflies feed on nectar from flowers.
After eggs are laid, the insectembryo develops and the eggs hatch. In some wingless insects, such as silverfish, development is direct; the eggs hatch into miniature forms that look just like tiny adults. These insects go through successive molts until the adult size is reached. Many other species of insects undergo a series of major changes in body structure as they develop. In some cases, the adult insect bears ittle resemblance to its juvenile stage. This series of changes, controlled by chemical- substances in the animal, is called metamorphosis.
Insects usually mate once during their lifetime. The eggs usually are fertilized internally. Some insects exhibit parthenogenesis, reproducing from unfertilized eggs. In aphids, parthenogenesis produces all-female generations. Most insects lay a large number of eggs, which increases the chances that some offspring will sur- vive long enough to reproduce. Many female insects are equipped with an appendage that can pierce through the surface of the ground or into wood. The female lays eggs in the hole.
Slugs, snails, squids, and some animals that live in shells in the ocean or on the beach are all mollusks. These organisms belong to the phylum Mollusca. Members of this phylum range from the slow moving slug to the jet-propelled squid. Although most species live in the ocean, others live in freshwater and moist terrestrial habitats. Some quatic mollusks, such as oysters, live much of their lives firmly attached to the ocean floor or to submerged docks or parts of boats. Others, such as he octopus, swim freely in the ocean. Land-dwelling slugs and snails can be found slowly moving over leaves on the forest floor.
Some mollusks have shells, and others, including slugs and squids, are adapted to life without a hard cover ing. All mollusks have bilateral symmetry, a coelom, a digestive tract with two openings, a muscular foot, and a mantle. The mantle is a membrane that surrounds the internal organs of the mollusk. In shelled mollusks, the mantle secretes the shell. Although mollusks look different from one another on the outside, they share many similarities on the inside. You can see the similarities and the differences in the snail and a squid in this picture.
Mollusks reproduce sexually and most have separate sexes. In most aquatic species, eggs and sperm are released at the same time into the water, where external fertilization takes place. Many gastropods that live on land, and a few bivalves, are hermaphrodites and produce both eggs and sperm. Fertilization is internal. Although members of the phylum Mollusca have different appearances as adults, they all share similar developmental patterns. One larval stage of most mollusks is similar. Some marine mollusks have free- swimming larvae that propel them- selves by cilia. Most marine snails and bivalves have another developmental stage called a veliger in which the beginnings of a foot, shell, and mantle can be seen.
The largest class of mollusks is Gastropoda, or the stomach-footed mollusks. The name comes from the way the animal’s large foot is positioned under the rest of its body. Most species of gastropods have a shell. Other gastropod species, such as slugs, have no shell. Shelled gastropods include snails, abalones, conches, periwinkles, whelks, limpets, cowries, and cones. They can be found in freshwater, saltwater, or moist terrestrial habitats. Shelled gastropods may be plant eaters, predators, or parasites. Instead of being protected by a shell, the body of a slug is protected by a thick layer of mucus. Colorful sea slugs, also called nudibranchs, are protected in another way. When certain species of sea slugs feed on jellyfishes, they incorporate the poisonous nematocysts of the jellyfish into their own tissues with out causing these cells to discharge. Any fishes trying to eat the sea slugs are repelled when the nematocysts discharge into the unlucky predator. The bright colors of these gastropods warn predators of the potential danger.
Two-shelled mollusks such as clams, oysters, and scallops belong to the class Bivalvia. Most bivalves are marine, but a few species live in freshwater habitats. Bivalves occur in a range of sizes. Some are less than 1 mm in length and others, such as the tropical giant clam, may be 1.5 m long. Bivalves have no distinct head or radula. Most use their large, muscular foot for burrowing in the mud or sand at the bottom of the ocean or a lake. A ligament, like a hinge, connects their two shells, called valves; strong muscles allow the valves to open and close over the soft body. See if you can identify the shells pic tured in the MiniLab by using the dichotomous key given. One of the main differences between gastropods and bivalves is that bivalves are filter feeders that obtain food by filtering small particles from the surrounding water. Bivalve mollusks have several adaptations for filter feeding, including gill cilia that beat to draw water in through an incurrent siphon. As water moves overthe gills, food and sediments become trapped in mucus. Cilia that line the gills push food particles to the mouth. Cilia also act as sorting devices. Large particles, sediment, and anything else that is rejected is transported to the mantle where it is expelled through the excurrent siphon, or to the foot, where it is eliminated from the animal’s body.
The head-footed mollusks are marine organisms in the class Cephalopoda. This class includes the ctopus, squid, cuttlefish, and chambered nautilus, as shown in Figure 27.9. The only cephalopod with a shell is the chambered nautilus, but some species, such as the cuttlefish, have a reduced internal shell. Scientists consider the cephalopods to have the most complex structures and to be the most recently evolved of all mollusks. In cephalopods, the foot has evolved into tentacles with suckers, hooks, or adhesive structures. Cephalopods C swim or walk over the ocean floor in pursuit of their prey, capturing it with their tentacles. Once tentacles have captured prey, it is brought to the mouth and bitten with beaklike jaws. Then the food is torn and pulled into the mouth by the radula. Like bivalves, cephalopods have siphons that expel water. These mollusks can expel water forcefully in any direction, and move quickly by jet propulsion. Squids can attain speeds of 20 m per second using this system of movement. You may be aware that cephalopods use jet propulsion to escape from danger. Squids and octopuses also can release a dark fluid to cloud the water. This “ink” helps to confuse their predators so they can make a quick escape.
Characteristics of Echinodermata:
1)Possess 5-rayed symmetry, mostly radial, sometimes bilateral.
2)Body has more than two cell layers, tissues and organs.
3)Body cavity a true coelom.
4)Most possesses a through gut with an anus.
5)Body shape highly variable, but with no head.
6)Nervous system includes a circum-oesophageal ring.
7)Has a poorly defined open circulatory system.
8)Possesses a water vascular system, which hydraulically operates the tube feet or feeding tentacles.
9)Without excretory organs.
10)Normally possesses a subepidermal system of calcareous plates
11)Reproduction normally sexual and gonochoristic.
12)Feeds on fine particles in the water, detritus or other animals.
13)All live marine environments.
The Echinodermata are Feather Stars, Starfish, Sea Urchins, Brittle Stars, Sea Cucumbers, Sand Dollars and Sea Lilies. As you can see from this list they are a morphologically very diverse group, at first it may not be obvious how they are all related, but despite their various forms they do all possess the characteristics outlined above. They are an ancient and very successful phylum of invertebrates with around 6,000 living members. They are one of the best known and most loved groups of invertebrates. They are popular as symbols because of their unique shapes and beautiful colours. They are also one of the most evolutionarily advanced phyla, yet they are totally unique in many ways. They have evolved many morphological and physiological characteristics that set them apart from all the other phyla, and although much research has been done on them, we are still a long way from really understanding them well. The echinoderms are, to express it simply, wonderfully attractive both aesthetically and intellectually.
The body wall of echinoderms consists of three layers. The outer layer, called the epidermis, is only a single layer of cells which covers the entire animal including its various spines. The third layer is also a single layer of cells the main difference being that these cells are ciliated. This layer encloses the the animal's coelom separating the animals guts from its skin. It is called the 'coelomic lining'. The middle layer is much thicker and is called the dermis. It is composed of connective tissue and contains the exoskeleton. This exoskeleton takes three different forms. 1) A set of closely joined plates with little individual movement that exist as a test or shell (Feather Stars and Sea Urchins). 2) A set of separately articulating (more freely moving) small plates called ossicles (Starfish, Brittle Stars and the arms of Crinoids). 3) A collection of widely separated microscopic ossicles lying in the dermis, which in this case is quite leathery (Sea Cucumbers). Whatever form they take these plates or ossicles are always made from calcite, the ingredients for which are found in sea water. The exoskeleton of echinoderms grows continuously throughout the animals life, thus older animals are always larger than younger ones. The exoskeleton supports the spines, warts and tubercles that are often found on the echinoderm surface. These various protuberances are also generally made from calcite.
Apart from its skin, and or its spines, an echinoderm also has contact with the external world through its water vascular system and the tube-feet that are a part of this system. The water vascular system of the echinoderms is unique in the living world and easily distinguishes them from all other phyla. The system takes slightly different forms in the different classes. In the Crinoidea, which are believed to be the most ancient of the echinoderms, the tube feet are branched and secrete mucous. In the Ophiuroidea the tube feet are simple and slender. In Asteroidea, Echinoidea and Holothuroidea they are thicker and end in suckers. The water vascular system starts with an opening to the external environment called a madreporite. From this a short straight canal called the 'stone canal' leads to the 'ring canal'. The ring canal is a ring as might be expected and it has five longitudinal canals branching off from it into each of the arms, or their morphological equivalents in Echinoidea and Holothuroidea. In species with more than 5 arms these canals branch out into each arm. on each side of each canal there arises a series of short lateral canals that lead, via a valve, into the descending tube feet and the ampulla that operate them. The tube feet pass through small holes in the animals exoskeleton and muscles around the ampulla (which remains inside the exoskeleton) squeeze water into them causing them to extend or relax.
Echinoderms are either filter feeders, substrate eaters or carnivores. The gut is U-shaped in the Crinoidea with the mouth and anus being on the same surface. In the other groups it is straight-through gut with the mouth and anus on approximately opposite sides of the body. Echinoderms have a sub-epidermal nerve net running all over their body. As well as this they have a circum-oral nerve ring with 5 radial nerve cords extending from it into the arms. These nerves are in connection with those of the sub-epidermal nerve net. Many Echinoderms use their tube feet as organs for gaseous exchange, but others such as the Ophiuroidea and the Holothuroidea have additional special sites or organs of respiration. Fertilisation is always external and the larvae are planktonic and biradial, they develop the pentaradial symmetry of the adult forms later on.
Many echinoderms are able to regenerate lost parts, and some, especially sea stars and brittle stars, drop various parts when under attack. In a few echinoderms, asexual reproduction takes place by splitting, and the broken parts of sea stars can sometimes regenerate whole animals. Some of the smaller brittle stars, especially tropical species, regularly reproduce by breaking into two equal parts; each half then re- generates a whole animal. Despite the ability of many echinoderms to break into parts and regenerate new animals from them, most reproduction in the phylum is sexual and external. The sexes in most echinoderms are separate, although there are few external differences. Fertilized eggs of echinoderms usually develop into free-swimming, bilaterally symmetrical larvae, which differ from the trochophore larvae of mollusks and annelids. These larvae form a part of the plankton until they metamorphose through a series of stages into the more sedentary adults. There are more than 20 extinct classes of echinoderms and an additional 6 with living members.
Sea stars, or starfish, are perhaps the most familiar echinoderms. Among the most important predators in many marine ecosystems, they range in size from a centimeter to a meter across. They are abundant in the intertidal zone, but they also occur at depths as great as 10,000 meters. Around 1500 species of sea stars occur throughout the world.The body of a sea star is composed of a central disc that merges gradually with the arms. Although most sea star have five arms, the basic symmetry of the phylum, members of some families have many more, typically in multiples of five. The body is somewhat flattened, flexible, and covered with a pigmented epidermis.
The mouth of a sea star is located in the center of its oral surface. Some sea stars have an extraordinary way of feeding on bivalve mollusks. They can open a small gape between the shells of bivalves by exerting a muscular pull on the shells. Eventually, muscular fatigue in the bivalve results in a very narrow gape, sufficient enough for the sea star to insert its stomach out through its mouth into the bivalve. Within the mollusk, the sea star secretes its digestive enzymes and digests the soft tissues of its prey, retracting its stomach when the process is complete.
Brittle stars are the largest class of echinoderms in numbers of species (about 2000) and they are probably the most abundant also. Secretive, they avoid light and are more active at night. Brittle stars have slender, branched arms. The most mobile of echinoderms, brittle stars move by pulling themselves along, “rowing” over the substrate by moving their arms, often in pairs or groups, from side to side. Some brittle stars use their arms to swim, a very unusual habit among echinoderms. Brittle stars feed by capturing suspended microplankton and organic detritus with their tube feet, climbing over objects on the ocean floor. In ad- dition, the tube feet are important sensory organs and assist in directing food into the mouth once the animal has captured it. As implied by their com- mon name, the arms of brittle stars detach easily, a characteristic that helps to protect the brittle stars from their predators. Like sea stars, brittle stars have five arms. More closely related to the sea stars than to the other classes of the phylum, on closer inspection they are surprisingly different. They have no pedicellariae, as sea stars have, and the groove running down the length of each arm is closed over and covered with ossicles. Their tube feet lack ampullae, have no suckers, and are used for feeding, not locomotion. Brittle stars usually have separate sexes, with the male and female gametes in most species being released into the water and fusing there. Development takes place in the plankton and the larvae swim and feed using elaborate bands of cilia. Some species brood their young in special cavities and fully developed juvenile brittle stars emerge at the end of development.
The members of the class Echinoidea, sand dollars and sea urchins, lack distinct arms but have the same five-part body plan as all other echinoderms. Five rows of tube feet protrude through the plates of the calcareous skeleton, and there are also openings for the mouth and anus. These different openings can be seen in the globular skeletons of sea urchins and in the flat skeletons of sand dollars. Both types of endoskeleton, often common along the seashore, consist of fused calcareous plates. About 950 living species constitute the class Echinoidea. Echinoids walk by means of their tube feet or their movable spines, which are hinged to the skeleton by a joint that makes free rotation possible. Sea urchins and sand dollars move along the sea bottom, feeding on algae and small fragments of organic material. They scrape these off the substrate with the large, triangular teeth that ring their mouths. The gonads of sea urchins are considered a great delicacy by people in different parts of the world. Because of their calcareous plates, sea urchins and sand dollars are well preserved in the fossil record, with more than 5000 additional species described. As with most other echinoderms, the sexes of sea urchins and sand dollars are separate. The eggs and sperm are shed separately into the water, where they fuse. Some brood their young, and others have free- swimming larvae, with bands of cilia extending onto their long, graceful arms.
Sea cucumbers (class Holothuroidea) are shaped somewhat like their plant namesakes. They differ from the preceding classes in that they are soft, slug like organisms, often with a tough, leathery outside skin. The class consists of about 1500 species found worldwide. Except for a few forms that swim, sea cucumbers lie on their sides t the bottom of the ocean. Their mouth is located at one end and is surrounded by eight to 30 modified tube feet called tentacles; the anus is at the other end. The tentacles around the mouth may secrete mucus, used to capture the small planktonic organisms on which the animals feed. Each tentacle is periodically wiped off within the esophagus and then brought out again, covered with a new supply of mucus. Sea cucumbers are soft because their calcareous skeletons are reduced to widely separated microscopic plates. These animals have extensive internal branching systems, called respiratory trees, which arise from the cloaca, oranal cavity. Water is pulled into and expelled from the respiratory tree by contractions of the cloaca; gas exchange takes place as this process occurs. The sexes of most cucumbers are separate, but some of them are hermaphroditic. Most kinds of sea cucumbers have tube feet on the body in addition to tentacles. These additional tube feet, which might be restricted to five radial grooves or scattered over the surface of the body, may enable the animals to move about slowly. On the other hand, sea cucumbers may simply wriggle along whether or not they have additional tube feet. Most sea cucumbers are quite sluggish, but some, especially among the deep-sea forms, swim actively. Sea cucumbers, when irritated, sometimes eject a portion of their intestines by a strong muscular contraction that may send the intestinal fragments through the anus or even rupture the body wall.
The most recently described class of echinoderms (1986), sea daisies are strange little discshaped animals, less than 1 cm in diameter, discovered in waters over 1000 m deep off New Zealand. Only two species are known so far. They have five part radial symmetry, but no arms. Their tube feet are located around the periphery of the disc, rather than along radial lines, as in other chinoderms. One species has a shallow, saclike stomach but no intestine or anus; the other species has no digestive tract at all. The surface of its mouth is covered by a membrane through which it apparently absorbs nutrients
Sea lilies and feather stars, or crinoids (class Crinoidea) differ from all other living echinoderms in that the mouth and anus are located on their upper surface in an open disc. The two structures are connected by a simple gut. These animals have simple excretory and reproductive systems and an exten- sive water-vascular system. The arms, which are the food-gathering structures of crinoids, are located around the margins of the disc. Different species of crinoids may have from 5 to more than 200 arms extending upward from their bodies, with smaller structures called pinnules branching from the arms. In all crinoids, the number of arms is initially small. Species with more than 10 arms add additional arms progressively during growth. Crinoids are filter feeders, capturing the microscopic organisms on which they feed by means of the mucus that coats their tube feet, which are abundant on the animals’ pinnules. Scientists that study echinoderms believe that the common ancestors of this phylum were sessile, sedentary, radially symmetrical animals that resembled crinoids. Crinoids were abundant in ancient seas, and were present when the Burgess Shale was deposited about 515 million years ago. More than 6000 fossil species of this class are known, in comparison with the approximately 600 living species.
1. A single, hollow nerve cord runs just beneath the dorsal surface of the animal. In vertebrates, the dorsal nerve cord differentiates into the brain and spinal cord.
2. A flexible rod, the notochord, forms on the dorsal side of the primitive gut in the early embryo and is present at some developmental stage in all chordates. The notochord is located just below the nerve cord. The notochord may persist throughout the life cycle of some chordates or be displaced during embryological development as in most vertebrates by the vertebral column that forms around the nerve cord.
3. Pharyngeal slits connect the pharynx, a muscular tube that links the mouth cavity and the esophagus, with the outside. In terrestrial vertebrates, the slits do not actually connect to the outside and are better termed pharyngeal pouches. Pharyngeal pouches are present in the embryos of all vertebrates. They become slits, open to the outside in animals with gills, but disappear in those lacking gills. The presence of these structures in all vertebrate embryos provides evidence to their aquatic ancestry.
4. Chordates have a postanal tail that extends beyond the anus, at least during their embryonic development. Nearly all other animals have a terminal anus.
All chordates have all four of these characteristics at some time in their lives. For example, humans have pharyngeal slits, a dorsal nerve cord, and a notochord as embryos. As adults, the nerve cord remains while the notochord is replaced by the vertebral column and all but one pair of pharyngeal slits are lost. This remaining pair forms the Eustachian tubes that connect the throat to the middle ear. A number of other characteristics also distinguish the chordates fundamentally from other animals. Chordates’ muscles are arranged in segmented blocks that affect the basic organization of the chordate body and can often be clearly seen in embryos of this phylum. Most chordates have an internal skeleton against which the muscles work. Either this internal skeleton or the notochord makes possible the extraordinary powers of locomotion that characterize the members of this group.
Vertebral column. In vertebrates, the notochord is replaced during the course of embryonic development by a bony vertebral column. The column is a series of bones that encloses and protects the dorsal nerve cord like a sleeve.
Head. In all vertebrates but the earliest fishes, there is a distinct and well-differentiated head, with a skull and brain. For this reason, the vertebrates are sometimes called the craniate chordates (Greek kranion, “skull”).
In addition to these two key characteristics, vertebrates differ from other chordates in other important respects:
. Neural crest. A unique group of embryonic cells called the neural crest contributes to the development of many vertebrate structures. These cells develop on the crest of the neural tube as it forms by an invagination and pinching together of the neural plate. Neural crest cells then migrate to various locations in the developing embryo, where they participate in the development of a variety of structures.
Internal organs. Among the internal organs of vertebrates, livers, kidneys, and endocrine glands are characteristic of the group. The ductless endocrine glands secrete hormones that help regulate many ofthe body’s functions. All vertebrates have a heart and a closed circulatory system. In both their circulatory and their excretory functions, vertebrates differ markedly from other animals.
Endoskeleton. The endoskeleton of most vertebrates is made of cartilage or bone. Cartilage and bone are specialized tissue containing fibers of the protein collagen compacted together. Bone also contains crystals of a calcium phosphate salt. Bone forms in two stages. First, collagen is laid down in a matrix of fibers along stress lines to provide flexibility, and then calcium minerals infiltrate the fibers, providing rigidity. The great advantage of bone over chitin as a structural material is that bone is strong without being brittle. The vertebrate endoskeleton makes possible the great size and extraordinary powers of movement that characterize this group.
The first vertebrates evolved in the oceans about 470 million years ago. They were jawless fishes with a single caudal fin. Many of them looked like a flat hot dog, with a hole at one end and a fin at the other. The appearance of a hinged jaw was a major advancement, opening up new food options, and jawed fishes became the dominant creatures in the sea. Their descendants, the amphibians, invaded the land. Salamander-like amphibians and other, much larger now-extinct amphibians were the first vertebrates to live successfully on land. Amphibians, in turn, gave rise to the first reptiles about 300 million years ago. Within 50 million years, reptiles, better suited than amphibians to living out of water, replaced them as the dominant land vertebrates. With the success of reptiles, vertebrates truly came to dominate the surface of the earth. Many kinds of reptiles evolved, ranging in size from smaller than a chicken to bigger than a truck. Some flew, and others swam. Among them evolved reptiles that gave rise to the two remaining great lines of terrestrial vertebrates, birds (descendants of the dinosaurs) and mammals. Dinosaurs and mammals appear at about the same time in the fossil record, 220 million years ago. For over 150 million years, dinosaurs dominated the face of the earth. Over all these centuries the largest mammal was no bigger than a cat. Then, about 65 million years ago, the dinosaurs abruptly disappeared, for reasons that are still hotly debated. In their absence, mammals and birds quickly took their place, becoming in turn abundant and diverse. The history of vertebrates has been a series of evolutionary advances that have allowed vertebrates to first invade the sea and then the land. This invasion was a staggering evolutionary achievement, involving fundamental changes in many body systems. Vertebrates are a diverse group, containing members adapted to life in aquatic habitats, on land, and in the air. There are eight principal classes of living vertebrates. Four of the classes are fishes that live in the water, and four are land-dwelling tetrapods, animals with four limbs. (The name tetrapod comes from two Greek words meaning “four-footed.”) The extant classes of fishes are the superclass Agnatha (the jawless fishes), which includes the class Myxini, the hagfish, and the class Cephalaspidomorphi, the lampreys; Chondrichthyes, the cartilaginous fishes, sharks, skates, and rays; and Osteichthyes, the bony fishes that are dominant today. The four classes of tetrapods are Amphibia, the amphibians;Reptilia, the reptiles; Aves, the birds; and Mammalia, the mammals.
Fishes and Sharks
Over half of all vertebrates are fishes. The most diverse and successful vertebrate group, they provided the evolutionary base for invasion of land by amphibians. In many ways, amphibians, the first terrestrial vertebrates, can be viewed as transitional fish out of water. In fact, fishes and amphibians share many similar features, among the host of obvious differences. The story of vertebrate evolution started in the ancient seas of the Cambrian Period (570 to 505 million years ago), when the first backboned animals appeared. Wriggling through the water, jawless and toothless, these first fishes sucked up small food particles from the ocean floor like miniature vacuum cleaners. Most were less than a foot long, respired with gills, and had no paired fins—just a primitive tail to push them through the water. For 50 million years, during the Ordovician Period (505 to 438 mil- lion years ago), these simple fishes were the only vertebrates. By the end of this period, fish had developed primitive fins to help them swim and massive shields of bone for protection. Jawed fishes first appeared during the Silurian Period (438 to 408 million years ago) and along with them came a new mode of feeding. Later, both the cartilaginous and bony fishes appeared.
From whale sharks that are 18 meters long to tiny cichlids no larger than your fingernail, fishes vary considerably in size, shape, color, and appearance. Some live in freezing Arctic seas, others in warm freshwater lakes, and still others spend a lot of time out of water entirely. However varied, all fishes have important characteristics in common:
1. Gills. Fishes are water-dwelling creatures and must extract oxygen dissolved in the water around them. They do this by directing a flow of water through their mouths and across their gills. The gills are composed of fine filaments of tissue that are rich in blood vessels. They are located at the back of the pharynx and are supported by arches of cartilage. Blood moves through the gills in the opposite direction to the flow of water in order to maximize the efficiency of oxygen absorption.
2. Vertebral column. All fishes have an internal skeleton with a spine surrounding the dorsal nerve cord, although it may not necessarily be made of bone. The brain is fully encased within a protective box, the skull or cranium, made of bone or cartilage.
3. Single-loop blood circulation. Blood is pumped from the heart to the gills. From the gills, the oxygenated blood passes to the rest of the body, then returns to the heart. The heart is a muscular tube- ump made of four chambers that contract in sequence.
4. Nutritional deficiencies. Fishes are unable to synthesize the aromatic amino acids and must consume them in their diet. This inability has been inherited by all their vertebrate descendants.
Although the method may vary, all fishes reproduce sexually. Fertilization and development is external in most fishes. Eggs and sperm can be released directly into the water, or deposited in more protected areas, such as on floating aquatic plants. Although most fishes produce large numbers of eggs at one time, hagfishes produce small numbers of relatively large eggs. Cartilaginous fishes have internal fertilization. Skates deposit fertilized eggs on the ocean floor. Some female sharks and rays carry developing young inside their bodies. Because these young fishes are well developed when they are born, they have an increased chance of survival. Most bony fishes have external fertilization and development. This type of external reproduction in fishes and some other animals is called spawning. During spawning, some female bony fishes, such as cod, produce as many as 9 million eggs, of which only a small percentage survive. In some bony fishes, such as guppies and mollies, fertilization and development is internal. Most fishes that produce millions of eggs provide no care for their offspring after spawning. In these species, only a few of the young survive to adulthood. Some fishes, such as the mouth-brooding cichlids, stay with their young after they hatch. When their young are threatened by predators, the parent fishes scoop them into their mouths for protection.
Lampreys and hagfishes belong to the superclass Agnatha. The skeletons of agnathans, as well as of sharks and their relatives, are made of a tough, flexible material called cartilage. Though they do not have jaws, they are voracious feeders. A hagfish, has a toothed mouth and feeds on dead or dying fishes. It can drill a hole into a fish and suck out the blood and insides. Parasitic lampreys use their sucker like mouths to attack other fishes. They use their sharp teeth to scrape away the flesh and then suck out the prey’s blood.
Sharks, skates, and rays belong to the class Chondrichthyes. These fishes, like agnathans, possess skeletons composed entirely of cartilage. Because living sharks, skates, and rays are similar to species that swam the seas more than 100 000 years ago, they are considered living fossils. Sharks are perhaps the most well-known predators of the oceans. Like sharks, most rays are predators and feed on or near the ocean floor. Rays have flat bodies and broad pectoral fins on their sides. By slowly flapping their fins up and down, rays can glide as they search for mollusks and crustaceans along the ocean floor. Some species of rays have sharp spines with poison glands on their long tails that they can use for defense. Other species of rays have organs that generate electricity, which can stun or kill both prey and predators.
1. Legs were necessary to support the body’s weight as well as to allow movement from place to place.
2. Lungs were necessary to extract oxygen from air. Even though there is far more oxygen available to gills in air than in water, the delicate structure of fish gills requires the buoyancy of water to support them and they will not function in air.
3. The heart had to be redesigned to make full use of new respiratory systems and to deliver the greater amounts of oxygen required by walking muscles.
4. Reproduction had to be carried out in water until methods evolved to prevent eggs from drying out.
5. Most importantly, a system had to be developed to prevent the body itself from drying out.
Amphibians solved these problems only partially, but their solutions worked well enough that amphibians have survived for 350 million years. Evolution does not insist on perfect solutions, only workable ones. Ichthyostega, the earliest amphibian fossil was found in a 370-million-year-old rock in Greenland. At that time, Greenland was part of the North American con- tinent and lay near the equator. For the next 100 million years, all amphibian fossils are found in North America. Only when Asia and the southern continents all merged with North America to form the supercontinent Pangaea did amphibians spread throughout the world. Ichthyostega was a strongly built animal, with four sturdy legs well supported by hip and shoulder bones. The shoulder bones no longer attached to the skull as in fish. The hipbones were braced against the backbone unlike in fish, so the limbs could support the animal’s weight. To strengthen the backbone further, long, broad ribs that overlap each other formed a solid cage for the lungs and heart. The rib cage was so solid that it probably couldn’t expand and contract for breathing. Instead, Ichthyostega obtained oxygen somewhat as a fish does, by lowering the floor of the mouth to draw air in, then raising it to push air down the windpipe into the lungs.
Unlike fishes, most amphibians go through the process of metamorphosis. Fertilized eggs hatch into tadpoles, the aquatic stage of most amphibians. Tadpoles possess fins, gills, and a two- chambered heart as seen in fishes. As tadpoles grow into adult frogs and toads, they develop legs, lungs, and a three-chambered heart. Young salamanders resemble adults, but, as aquatic larvae, they have gills and usually have a tail fin. Most adult salamanders lack gills and fins. They breathe through their moist skin or with lungs. Salamanders in the family Plethodontidae have no lungs and breathe only through their skin. Completely terrestrial salamander species do not have a larval stage; the young hatch as smaller versions of adults. Most salamanders have four legs for moving about, but a few have only two front legs.
. Frogs and toads, amphibians without tails, live in a variety of environments from deserts and mountains to ponds and puddles. Frogs have smooth, moist skin, a broad body, and long hind legs that make them excellent jumpers. Most frogs live in or near water, although some tropical species live in trees. Unlike frogs, toads have a dry, bumpy skin, short legs, and are well adapted to dry environments. All adult anurans are carnivores, eating a wide variety of invertebrates. Most frogs and toads return to water to reproduce, laying their eggs directly in water. Their eggs lack water-tight external membranes and would dry out quickly out of the water. Eggs are fertilized externally and hatch into swimming larval forms called tadpoles. Tadpoles live in the water, where they generally feed on minute algae. After considerable growth, the body of the tadpole gradually changes into that of an adult frog. This process of abrupt change in body form is called metamorphosis.
. Salamanders have elongated bodies, long tails, and smooth moist skin. They typically range in length from a few inches to a foot, although giant Asiatic salamanders of the genus Andrias are as much as 1.5 meters long and weigh up to 33 kilograms. Most salamanders live in moist places, such as under stones or logs, or among the leaves of tropical plants. Some salamanders live entirely in water. Salamanders lay their eggs in water or in moist places. Fertilization is usually external, although a few species practice a type of internal fertilization in which the female picks up sperm packets deposited by the male. Unlike anurans, the young that hatch from salamander eggs do not undergo profound metamorphosis, but are born looking like small adults and are carnivorous.
All living reptiles share certain fundamental characteristics, features they retain from the time when they replaced amphibians as the dominant terrestrial vertebrates. Among the most important are:
1. Amniotic egg. Amphibians never succeeded in becoming fully ter- restrial because amphibian eggs must be laid in water to avoid dry- ing out. Most reptiles lay water- tight eggs that contain a food source (the yolk) and a series of four membranes—the yolk sac, the amnion, the allantois, and the chorion (figure 48.22). Each mem- brane plays a role in making the egg an independent life-support system. The outermost membrane of the egg is the chorion, which lies just beneath the porous shell. It allows respiratory gases to pass through, but retains water within the egg. Within, the amnion en- cases the developing embryo within a fluid-filled cavity. The yolk sac provides food from the yolk for the embryo via blood vessels con- necting to the embryo’s gut. The al- lantois surrounds a cavity into hich waste products from the embryo are excreted. ll modern reptiles (as well as birds and mammals) how exactly this same pattern of membranes within the egg. These three classes are called amniotes.
2. Dry skin. Living amphibians have a moist skin and must remain in moist places to avoid drying out. Reptiles have dry, watertight skin. A layer of scales or armor covers their bodies, preventing water loss. These scales develop as surface cells fill with keratin, the same protein that forms claws, fingernails, hair, and bird feathers.
3. Thoracic breathing. Amphibians breathe by squeezing their throat to pump air into their lungs; this limits their breathing capacity to the volume of their mouth. Reptiles developed pul- monary breathing, expanding and contracting the rib cage to suck air into the lungs and then force it out. The capacity of this system is limited only by the vol- ume of the lungs.
Turtles. The most ancient surviving lineage of reptiles is that of turtles. Turtles have anapsid skulls much like those of the first reptiles. Turtles have changed little in the past 200 million years. Turtles and Tortoises. The order Chelonia consists of about 250 species of turtles (Most of whish aquatic) and tortoises (which are terrestrial). They differ from all other reptiles because their bodies are encased within a protective shell. Many of them can pull their head and legs into the shell as well, for total protection from predators. Turtles and tortoises lack teeth but have sharp beaks. Today’s turtles and tortoises have changed very little since the first turtles appeared 200 million years ago. Turtles are anapsid—they lack the temporal openings in the skull characteristic of other living reptiles, which are diapsid. This evolutionary stability of turtles may reflect the continuous benefit of their basic design—a body covered with a shell. In some species, the shell is made of hard plates; in other species, it is a covering of tough, leathery skin. In either case, the shell consists of two basic parts. The carapace is the dorsal covering, while the plastron is the ventral portion. In a fundamental commitment to this shell architecture, the vertebrae and ribs of most turtle and tortoise species are fused to the inside of the carapace. All of the support for muscle attachment comes from the hell. While most tortoises have a domed-shaped shell into which they can retract their head and limbs, ater-dwelling turtles have a streamlined, disc-shaped shell that permits rapid turning in water. Freshwater turtles have webbed toes, and in marine turtles, the forelimbs have evolved into flippers. Although marine turtles spend their lives at sea, they must return to land to lay their eggs. Many species mi- grate long distances to do this. Atlantic green turtles mi- grate from their feeding grounds off the coast of Brazil to Ascension Island in the middle of the South Atlantic—a distance of more than 2000 kilometers—to lay their eggs on the same beaches where they hatched.
The third lineage of surviving reptiles to evolve were the Rhynchocephalonts, small diapsid reptiles that appeared shortly before the dinosaurs. They lived throughout the time of the dinosaurs and were common in the Jurassic. They began to decline in the Cretaceous, apparently unable to compete with lizards, and were already rare by the time dinosaurs disappeared. Today only two species of the order Rhynchocephalia survive, both tuataras living on small islands near New Zealand
Tuatara. The order Rhynchocephalia contains only two species today, the tuataras,large, lizard like animals about half a meter long. The only place in the world where these endangered species are found is on a cluster of small islands off the coast of New Zealand. The native Maoris of New Zealand named the tuatara for the conspicuous spiny crest running down its back. An unusual feature of the tuatara (and some lizards) is the inconspicuous “third eye” on the top of its head, called a parietal eye. Concealed under a thin layer of scales, the eye has a lens and retina and is connected by nerves to the brain. Why have an eye, if it is covered up? The parietal eye may function to alert the tuatara when it has been ex- posed to too much sun, protecting it against overheating. Unlike most reptiles, tuataras are most active at low temperatures. They burrow during the day and feed at night on insects, worms, and other small animals.
Lizards and snakes. Most reptiles living today belong to the second lineage to evolve, the lizards and snakes. Lizards and snakes are descended from an ancient lineage of lizard like reptiles that branched off the main line of reptile evolution in the late Permian, 250 million years ago, before the thecodonts appeared. Throughout the Mesozoic era, during the dominance of the dinosaurs, these reptiles survived as minor elements of the landscape, much as mammals did. Like mammals, lizards and snakes became diverse and common only after the dinosaurs disappeared.
: Lizards and Snakes. The order Squamata consists of three suborders: Sauria, some 3800 species of lizards, Amphisbaenia, about 135 species of worm lizards, and Serpentes, about 3000 species of snakes. The distinguishing characteristics of this order are the presence of paired copulatory organs in the male and a lower jaw that is not joined directly to the skull. A movable hinge with five joints (your jaw has only one) allows great flexibility in the movements of the jaw. In addi- tion, the loss of the lower arch of bone below the lower opening in the skull of lizards makes room for large mus- cles to operate their jaws. Most lizards and snakes are carnivores, preying on insects and small animals, and these improvements in jaw design have made a major contribution to their evolutionary success. The chief difference between lizards and snakes is that most lizards have limbs and snakes do not. Snakes also lack movable eyelids and external ears. Lizards are a more an- cient group than modern snakes, which evolved only 20 million years ago. Common lizards include iguanas, chameleons, geckos, and anoles. Most are small, measuring less than a foot in length. The largest lizards belong to the monitor family. The largest of all monitors is the Komodo dragon of Indonesia, which reaches 3 meters in length and weighs up to 100 kilograms. Snakes also vary in size from only a few inches long to those that reach nearly 10 meters in length. Lizards and snakes rely on agility and speed to catch prey and elude predators. Only two species of lizard are venomous, the Gila monster of the southwestern United States and the beaded lizard of western Mexico. Similarly, most species of snakes are nonvenomous. Of the 13 families of snakes, only 4 are venomous: the elapids (cobras, kraits, and coral snakes); the sea snakes; the vipers (adders, bush- masters, rattlesnakes, water moccasins, and copperheads);and some colubrids (African boomslang and twig snake). Many lizards, including skinks and geckos, have the abil- ity to lose their tails and then regenerate a new one. This apparently allows these lizards to escape from predators.
Crocodiles. The fourth lineage of living reptile, crocodiles, appeared on the evolutionary scene much later than other living reptiles. Crocodiles are descended from the same line of thecodonts that gave rise to the dinosaurs and resemble dinosaurs in many ways. They have changed very little in over 200 million years. Crocodiles, pterosaurs, thecodonts, and dinosaurs together make up a group called archosaurs (“ruling reptiles”).
Crocodiles and Alligators. The order Crocodilia is composed of 25 species of large, primarily aquatic, primitive-looking reptiles. In addition to crocodiles and alligators, the order includes two less familiar animals: the caimans and gavials. Crocodilians have remained relatively unchanged since they first evolved. Crocodiles are largely nocturnal animals that live in or near water in tropical or subtropical regions of Africa, Asia, and South America. The American crocodile is found insouthern Florida and Cuba to Columbia and Ecuador. Nilecrocodiles and estuarine crocodiles can grow to enormous size and are responsible for many human fatalities each year. There are only two species of alligators: one living in the southern United States and the other a rare endangered species living in China. Caimans, which resemble alligators, are native to Central America. Gavials are a group of fish- eating crocodilians with long, slender snouts that live only in India and Burma. All crocodilians are carnivores. They generally hunt by stealth, waiting in ambush for prey, then attacking ferociously. Their bodies are well adapted for this form of hunting: their eyes are on top of their heads and their nostrils on top of their snouts, so they can see and breathe while lying quietly submerged in water. They have enormous mouths, studded with sharp teeth, and very strong necks. A valve in the back of the mouth prevents water from entering the air passage when a crocodilian feeds underwater. Crocodiles resemble birds far more than they do other living reptiles. Alone among living reptiles, crocodiles care for their young (a trait they share with at least some dinosaurs) and have a four-chambered heart, as birds do. There are also many other points of anatomy in which crocodiles differ from all living reptiles and resemble birds. Why are crocodiles more similar to birds than to other living reptiles? Most biologists now believe that birds are in fact the direct descendants of dinosaurs. Both crocodiles and birds are more closely related to dinosaurs, and each other, than they are related to lizards and snakes.
Modern birds lack teeth and have only vestigial tails, but they still retain many reptilian characteristics. For instance, birds lay amniotic eggs, although the shells of bird eggs are hard rather than leathery. Also, reptilian scales are present on the feet and lower legs of birds. What makes birds unique? What distinguishes them from living reptiles?
1. Feathers. Feathers are modified reptilian scales that serve two functions: providing lift for flight and conserving heat. The structure of feathers combines maximum flexibility and strength with minimum weight. Feathers develop from tiny pits in the skin called follicles. In a typical flight feather, a shaft emerges from the follicle, and pairs of vanes develop from its opposite sides. At maturity, each vane has many branches called barbs. The barbs, in turn, have many projections called barbules that are equipped with microscopic hooks. These hooks link the barbs to one another, giving the feather a continuous surface and a sturdy but flexible shape. Like scales, feathers can be replaced. Feathers are unique to birds among living animals. Recent fossil finds suggest that some dinosaurs may have had feathers.
2. Flight skeleton. The bones of birds are thin and hollow. Many of the bones are fused, making the bird skeleton more rigid than a reptilian skeleton. The fused sections of backbone and of the shoulder and hip girdles form a sturdy frame that anchors muscles during flight. The power for active flight comes from large breast muscles that can make up 30% of a bird’s total body weight. They stretch down from the wing and attach to the breastbone, which is greatly enlarged and bears a prominent keel for muscle attachment. They also attach to the fused collarbones that form the so-called “wishbone.” No other living vertebrates have a fused collarbone or a keeled breastbone.
Birds, like reptiles, reproduce by internal fertilization and lay amniotic eggs usually inside a nest. Bird eggs are encased in a hard shell, unlike the leathery shell of a reptile. Bird nests may be made out of bits of straw and twigs, may consist of a depression scratched into the sand, or may be elaborate structures that are added to yearly. Whatever the type of nest, birds do not leave the eggs to hatch on their own. Instead, birds incubate or sit on their eggs to keep them warm. The eggs are turned periodi- cally so that they develop properly. In some species of birds, both parents take turns incubating eggs; in others, only one parent does so. Bird eggs are distinctive, and often the species of bird can be identified just by the color, size, and shape of an egg.
Unlike reptiles, which take on a wide variety of forms from legless snakes to shelled turtles, birds are all very much alike in their basic form and structure. You have no difficulty recognizing a bird. In spite of the basic uniformity of birds, they do exhibit specific adaptations, depending on the environment in which they live and the food they eat, ptarmigans have feathered legs and feet that serve as snowshoes in the winter, making it easier for the birds to walk in the snow. Penguins are flightless birds with wings and feet modified for swimming and a body surrounded with a thick layer of insulating fat. Large eyes, an acute sense of hearing, and sharp claws make owls well-adapted, nocturnal predators able to swoop with absolute precision onto their prey. The shape of a bird’s beak or bill gives clues to the kind of food the bird eats. Hummingbirds, for example, have long beaks that are used for obtaining nectar from flowers. Hawks, have curved beaks that are adapted for tearing apart their prey. Pelicans have huge bills with pouches that they use as nets for capturing fish. The short, stout beak of a cardinal is adapted to cracking seeds.
Mammals, like birds, are endotherms. The ability to maintain a fairly constant body temperature enables mammals to live in almost every possible environment on Earth. Mammals have important characteristics not found in other animals. They have hair and produce milk to nurse their young. Mammals also have diaphragms, four-chambered hearts, pecialized teeth, modified limbs, and highly developed brains. Mammals have hair Have you ever heard someone complain about a pet that is hedding its hair? There’s no doubt that such a pet is a mammal because hair is present on all mammals at some point in their lives. Like feathers, mammalian hair, made out of the protein keratin, is also thought o have evolved from scales. The arrangement of hair provides insulation and waterproofing and thereby conserves body heat. If you have ever worn a wool sweater made from the hair of a sheep, you know ow warm wool can be on a cold day. Hair also serves other functions. Although hair helps retain body heat, mammals also have internal feed- back mechanisms that signal the body to cool off when it gets too arm. Mammals cool off by panting and through the action of sweat glands. Panting releases water from the nose and mouth, which results in a loss of body heat. Sweat glands help regulate body temperature by secreting moisture onto the surface of the skin. As the moisture evaporates, it transfers heat from the body to the surrounding air.
Mammals have several types of glands, which are a group of cells that secrete fluids. They include glands that produce saliva, sweat, oil, digestive enzymes, hormones, milk, and scent. sweat glands help keep a mammal cool. Mammals also feed their young from mammary glands, possibly modified sweat glands, which produce and secrete milk, a liquid that is rich in fats, sugars, proteins, minerals, and vitamins. Mammals nurse their young until they are able to digest and absorb nutrients from solid foods.
One reason mammals are successful is that they guard their young fiercely and teach them survival skills. Mam- mals can accomplish complex behav- iors, such as learning and remembering what they have learned. Have you ever attended an aquarium show or watched a movie about per- forming dolphins and whales? Dol- phins exhibit a wide variety of learned behaviors, including the behaviors per- formed for films or in aquarium shows. Primates, including humans, are perhaps the most intelligent animals. Chimpanzees, for example, can use tools, work machines, and use sign language to communicate with humans. Mammalian intelligence is a result of complex nervous systems and highly developed brains. The outer layer of a mammalian brain often is folded, forming ridges and grooves. These ridges and grooves increase the brain’s active surface area.
Marsupials make up the second subclass of mammals. A marsupial is a mammal in which the young have a short period of development within the mother’s body, followed by a period of development inside a pouch made of skin and hair on the outside of the mother’s body. You may have seen the only North American marsupial, the opossum. Most marsupials are found in Australia and surrounding islands. The theory of plate tectonics explains why most marsupials are found in Australia today. Scientists have found fossil marsupials on the continents that once made up Gondwana. These fossils support the idea that marsupials originated in South America, moved across Antarctica, and populated Australia before Gondwana broke up. Ancestors of today’s marsupials were able to populate the landmass that became Australia without having to share the area with the competitive placental mammals that evolved in other places. They successfully spread out and filled niches similar to thosethat placental mammals filled in all other parts of the world, since humans introduced sheep, rabbits, and other placental mammals to Australia, many of the native marsupial species have become threatened, endangered, or even extinct.
Do you think the animal shown in Figure 32.9B is a mammal? It has hair and mammary glands, yet it lays eggs. The duck-billed platypus is a monotreme (MA nuh treem), a mam- mal that reproduces by laying eggs Spiny anteaters, also called echidnas, belong to this subclass as well. Monotremes are found only in Aus- tralia, Tasmania, and New Guinea. One of the two species of spiny anteaters can be found only in New Guinea. Only three species of mono- tremes are alive today. The platypus, a mostly aquatic ani- mal, has a broad, flat tail, much like that of a beaver. Its rubbery snout resembles the bill of a duck. The platypus has webbed front feet for swimming through water, but it also has sharp claws on its front and hind feet for digging and burrowing into the soil. Much of its body is covered with thick, brown fur. The spiny anteater has coarse, brown hair, and its back and sides are covered with sharp spines that it can erect for defensive purposes when threatened by enemies. From its mouth, the anteater extends its long, sticky tongue to catch insects.
Humans, apes, monkeys, and lemurs belong to a group of mammals called primates. Though primates are highly diverse, they share some general features. Some primates have a high level of manual dexterity, which is the ability to manipulate or grasp objects with their hands. They usually also have keen eyesight and long, highly movable arms. Compared to other animals, they have large brains. The rimates with the largest brains, which includes humans, have the capacity to reason.
Primates are distinguished by their flexible hands and feet. All primates typically have five digits on each hand and oot; as you know, humans have fingers and toes. Most have flat nails and sensi-tive areas on the ends of their digits. The first digits on most primates’ hands are opposable, and the first digit on many primates’ feet are opposable. An opposable first digit, either a thumb or a great toe, is set apart from the other digits. This digit can be brought across the alm or foot so that it touches or nearly touches the other digits. This action allows the primate to grasp an object in a powerful grip. Some primatesal so have lengthened first digits that provide added dexterity.
Most primates have fewer offspring than other animals. Usually, primates give birth to one offspring at a time. Compared to other mammals, pregnancy is long, and newborns are dependent on their mothers for an extended period of time. For many primates, this time period allows for the increased learning of complex social interactions. A low reproductive rate, the loss of tropical habitats, and human predation has threatened some primate opulations. Many are endangered.