AEBI211 - Metazoan tree of life

Coelom formation

Schizocoely:

• Band of mesoderm forms around the gut before a coelom forms
• Coelom is formed by mesodermal cells dividing

Enterocoely:

• Mesoderm and coelom form at the same time
• Gastrulation begins, one side of the blastula bends inward (invagination) forming the archenteron (gastrocoel, 'gut cavity') → archenteron elongates → sides push outward and expand into a pouch-like coelomic compartment → pouch-like compartment pinches off

Body plans

Acoelomate:

• Mesoderm fills blastocoel (i.e. mesoderm completely fills body cavity)

Pseudocoelomate:

• Mesoderm lines only the outside of the blastocoel (ectoderm)
• Does not touch the gut
• Presence of a pseudocoel (an "empty" cavity)

Eucoelomate (aka coelomate):

• Band of mesoderm surrounds gut and then splits open
• Mesoderm completely lines the coelom
• True coelom

→ Schizocoely can form all 3

→ Enterocoely can only form coelomate

Asymmetrical

Asymmetrical

No symmetry

ex:

• Some sponges
• Most protozoans (but not all, like amoebas)

ex: Sponge (Porifera)

Spherical

• Ball shaped
• Any plane passing through the center divides the body into mirrored halves
• Best suited for floating and rolling

ex: Some protozoans

ex: Radiolaria (protozoa)

Radial

• Both radial and biradial are associated with just drifting through environment
• Tube- or vase-like
• Body divided into similar halves by more than 2 planes passing through the longitudinal axis
• Can interact with the environment in all directions

ex: Sponges, jellyfish, sea urchins

ex: Moon jelly (Cnidaria)

Biradial

• Associated with drifting through environment
• Radially symmetrical with the exception of a paired structure

Biradial

ex: Comb jellies (Ctenophora)

Bilateral

ex: Sea turtle (Chordata)

Bilateral

• Right and left sides (mirror images)
• Better for directional (forward) movement
• Associated with cephalization
• Often has mouth in front to allow for more efficient feeding and detection of prey

ex: Most metazoans

ex: Paramecium sp.

Protoplasmic

• Unicellular organisms
• All life functions confined within a single cell

ex: Protozoans

Cellular

Can be...

Colonial:

• Aggregation of undifferentiated cells
• ex: Choanoflagellates (Protozoans)

Multicellular:

• Aggregation of cells that are functionally different
• ex: Sponges (Porifera)

Cell-tissue

• Cells organized into tissues
• Cells aggregate into patterns or layers

Tissue = group of similar cells organized to perform a common function

• True tissue = secretes extracellular matrix in form of a basement membrane on which cells sit

ex: Cnidarians, some sponges

Tissue-organ

• Tissues organized into organs
• Organs contain more than one type of tissue (ex: gonads, lungs, heart)
• More specialized function

ex: Platyhelminthes

Organ-system

• Organs organized into systems
• Organs work together in a system (ex: reproductive, respiratory, circulatory, skeletal)

ex: All other eukaryotes

Undulipodia

ex: Paramecium with cilia

Undulipodia

Cilia and flagella are morphologically the same:

→ 9 pairs of microtubules arranged around a central pair

Cilia:

• Hairlike organelle found on many animal cells
• May be used is moving particles along the cell surface, or for locomotion
• Numerous
• Propel water parallel to the cell surface

Flagella:

• Whiplike organelle of locomotion
• Long hair-like structure / tail
• Propels water parallel to the flagellum axis

ex: Euglena with single flagellum

Pseudopodia

• Temporary projections of cell membrane
• Used for locomotion and phagocytosis

Cytoplasm is not homogenous:

• Ectoplasm: semi-solid outer layer
• Endoplasm: inner fluid

→ Endoplasm can turn into ectoplasm, vice-versa. This is used in pseudopodia.

→ Endoplasm flows forward into pseudopod and solidifies into ectoplasm

Holozoic vs Saprozoic

Phagocytosis:

1. Plasma membrane folds around the food particle

2. Membrane is pinched off at the surface

3. Food particle is in an intracellular membrane-bound vesicle (food vacuole or phagosome)

4. Lysosomes (small vesicles containing digestive enzymes) fuse with the food vacuole and pour the content into it

5. Digestion begins

Holozoic vs Saprozoic

Holozoic feeders:

• Ingest visible particles of food
• i.e. phagocytosis

Saprozoic feeders:

• Ingest food in a soluble form (soluble food moves directly across membrane, i.e. diffusion)

Symbiosis: Termites & Prokaryotes

ex: Lignocellulose digestion in termites

Termite

• Phylum Arthropoda
• Class Insecta

Mutualistic symbiosis: gut is colonized by flagellate protozoans that digest lignocellulose for the termite

Paramecium sp.

ex: Reproduction in Paramecium sp.

Paramecium are multinucleate (at least one macronucleus and one micronucleus)

• Macronucleus:
• Metabolism, synthesis, development
• Cannot survive without macronucleus
• Micronucleus:
• Sexual reproduction
• Cannot reproduce without the micronucleus

Apicomplexa

ex: Reproduction in Apicomplexa

• Phylum of parasitic protists
• All endoparasites
• Hosts include many animal phyla
• No obvious, unifying, locomotor organelles
• Life cycle includes both asexual and sexual reproduction
• Sometimes use an intermediate host

Asconoid

• Flagellated spongocoel (i.e. lined with choanocytes)
• Water enters through ostia into spongocoel, exits through single large osculum
• Size is limited (large spongocoel would create 'dead' space)
• Most simple form

Syconoid

• Flagellated canals
• Wall of sponge invaginated to form canals
• Incurrent canals
• Radial canals
• Water flows in through incurrent canals, through prosopyles into radial canals, through apopyles into spongocoel, and exits through osculum
• Bigger body surface

Leuconoid

• Flagellated chambers
• Most complex
• Numerous oscula, but no spongocoel
• Incurrent canals connect ocean to chambers, and excurrent canals connect chambers to the oscula
• Increased efficiency, can grow bigger

Pinacocytes

• Epithelial type cells (like skin)
• Closest thing to a tissue in a sponge
• Protective and contractile

Porocytes

• Pore cells
• Only is asconoid sponges

Choanocyte

Choanocytes

• Flagellated collar cells
• One end embedded in mesohyl, other end exposed
• Exposed end: flagellum surrounded by a collar of microvilli
• Collar = filter for food particles
• Food then ingested by phagocytosis
• Move water + collect food particles to consume by phagocytosis

Archaeocytes

• Amoeboid cells
• Can move through mesohyl
• Receive particles for digestion from choanocytes
• Transport food and oxygen to other cells

Spongin

• Form of collagen secreted by Class Demospongiae
• Forms skeletal network of some sponges

Spongin

Spicules

• Made of silica or calcium
• Many different shapes
• Can be used to classify sponges

Spicules

Vivipary:

Sexual reproduction

1. Sperm released into water

2. Taken up into canal system of other individual

3. Choanocytes phagocynthesize the sperm

4. Carry sperm through the mesohyl to the oocytes

5. Zygote retained by parents

6. Ciliated larva released

Sexual

• Sperm and oocytes develop from choanocytes (or sometimes archaeocytes)
• Mostly viviparous, some oviparous
• Indirect development

Ovipary:

Oocytes and sperm both released into water.

Asexual reproduction

Asexually reproduce by:

• Fragmentation
• Budding
• Gemmulation

Asexual

Polyp form

• Sedentary (inactive, little movement) or sessile (fixed in one place, immobile)
• Tube shaped
• Mouth surrounded by tentacles
• Can be attached to a substratum (solid surface) by pedal disk

Medusa form

• Aka jellyfishes or jellies
• Floating or free-swimming
• Umbrella-shaped
• Mouth centered on concave side
• Tentacles extend from rim of umbrella shape

Sperm fertilizes egg in gastric pouch

Ephyrae grow into mature jellyfish (medusa form)

Zygote develops on arms of female

Life cycle of Aurelia (moon jelly)

Releases saucerlike buds called ephyrae (strobilation)

ex: Life cycle of moon jelly

Planula larva attach to substratum → becomes a scyphistoma (polyp from)

Becomes a strobila

Scyphistoma can bud to form other polyps (asexual reproduction)

Class Hydrozoa

ex: Hydra sp.

Hydrozoa

• Hydroids, fire corals, Portuguese

man-of-war

• Life cycle typically includes asexual polyp and sexual medusa stages
• Freshwater hydra (lab) have no medusa stage
• Others have no polyp stage, occur only as medusa

ex: Portuguese man-of-war colony

• Order Siphonophora
• Made up of many individuals
• 3 types of polyps (stinging tentacles, reproduction, digestion) + a float
• Very toxic sting

ex: Portuguese man-of-war

Class Scyphozoa

Scyphozoa

• True jellyfish
• Dioecious
• Internal fertilization
• Exhibit both medusa and polyp stage during life cycle
• Medusa reproduces sexually
• Polyp reproduces asexually

ex: Moon jellyfish

Class Cubozoa

Cubozoa

• "Box jellyfish"
• Prominently medusa form (polyp form not yet identified)
• Medusa bell is almost square (cube)
• Tentacles at the corners of the square
• Strong swimmers and predators
• Stings from some species can be fatal to humans

Class Anthozoa

Anthozoa

• Largest class
• Polyps with a flower-like appearance
• No medusa stage
• Sexual and asexual reproduction
• All marine
• Solitary or colonial

ex: Sea anemones, corals, sea fans

Proglottids fertilized by another proglottid on same or different strobila

Sexual reproduction

Shelled embryos form in the uterus of the proglottid

Reproduction

• Nearly all monoecious
• Strobilation:
• Repeated transverse segmentation
• Ex: proglottids of tapeworms, and ephyrae of scyphozoan jellyfish
• Asexual: Proglottids can break off and become new individual

Strobilation (new proglottids form behind the scolex)

Terminal gravid proglottids break off and are excreted in hosts feces

Trematode eggs passed in bird's feces

Swimmer's itch

Eggs hatch and liberate miracidia (free-swimming, ciliated larvae)

Example: Swimmer's itch

Cercariae penetrate skin of birds, migrate to blood vessels to complete life cycle

• Caused by trematode, genus Shistosoma
• Normally passes between snails (intermediate host) and birds (definitive host)
• Cercariae can enter skin of humans
• Cercariae = form that emerges from snail after asexual reproduction, tadpolelike juveniles
• Humans = dead-end for flukes' lifecycles (can't develop further in human host)
• Very itchy!

Parasite develops in a molluscan intermediate host (ex: snail)

Cercariae leave intermediate host

Humans exposed to cercariae

Head-foot

Head:

• Well-developed head with mouth and sensory organs (exception: bivalves)
• Radula (unique to molluscs)
• Rasping, protrusible, tonguelike organ
• In most molluscs (except some bivalves and some gastropods)
• Like a tongue with teeth on it

Foot:

• Adapted for: locomotion and/or attachment
• Usually a ventral, sole-like structure
• Modifications (examples):
• Bivalves: laterally compressed foot
• Cephalopods: funnel (siphon) used for jet

propulsion

Mantle

• Sheath of skin
• Extending dorsally from the visceral mass
• Warps around each side of the body
• Protects soft parts
• Outer surface of the mantle secretes a shell
• Mantle cavity:
• Houses respiratory organs (gills or a lung)
• + Mantle's own exposed surface participates in gas exchange
• Where products from digestive, excretory, and reproductive systems empty

Gastropoda

Gaster (stomach) + podos (foot)

Gastropoda

• > 70,000 living species
• Many (but not all) have shells
• Terrestrial or aquatic
• Only molluscs to exploit terrestrial environment
• Bilaterally symmetrical, but visceral mass is asymmetrical
• Due to adaptation to avoid fouling: gill and kidney on the right side have been lost (water flows one-way: in the left side, over the gill, and out the right side, clearing waste from the rectum)

ex: Snails, slugs, sea slugs, sea butterflies

Bivalvia

• Two valves (i.e. shells)
• Marine and freshwater forms
• Mostly sedentary filter feeders
• Draw water through gills by ciliary action
• No head, no radula
• Very little cephalization (only a slight concentration of neurons, no actual head)
• Sensory organs: Ocelli
• Simple eye or eyespots
• Some bivalves (ex: scallops) have them
• Can sense light

ex: Mussels, clams, scallops, oysters

Cephalopoda

Kephale (head) + pods (foot)

Cephalopoda

• Most complex molluscs
• Marine active predators (eat small fishes, other molluscs, crustaceans, and worms)
• Highly mobile, rapid swimmers
• Swim by ejecting a jet of water from their mantle cavity through their funnel (modification of foot)
• Tentacles and arms capture prey by adhesive secretions or by suckers
• Octopuses and cuttlefish: have salivary glands that secrete a venom for immobilizing prey
• Vary in size (2 cm - 18 m → Giant squid)

ex: Squids, octopuses, nautiluses,

devilfish, cuttlefish

Earthworms

• Burrow in moist, rich soil
• Emerge at night to feed on surface detritus and vegetation + to breed
• In damp weather, they stay near the surface (sometimes with mouth or anus protruding)

Example: Earthworms

Metamerism in Earthworm

Example: Earthworm

• Some structures (gut, principle blood vessels, nerves) extend entire length of body
• Gonads are repeated in each or a few segments
• Nerves, blood vessels, and excretory organs are found in each metamere

Pseudometamerism

pseudo = false

Pseudo-metamerism

Repeated segments are independent of each other.

i.e. each segment contains a complete set of organs.

Ex: Tapeworms

• Proglottids break off and are shed in the hosts feces

Chelicerata

• 2 tagmata: cephalothorax + abdomen
• Most have 6 pairs of cephalothoracic appendages:
• 1 pair of chelicerae (mouthparts)
• 1 pair of pedipalps
• 4 pairs of walking legs

ex: Spiders, tick and mites, horseshoe crabs, scorpions, etc.

Myriapoda

Myrias (a myriad) + podos (foot)

• 2 tagmata: head + trunk
• Paired appendages on most trunk segments

ex: Millipedes, centipedes

Myriapoda

Crustacea

Crusta (shell)

Crustacea

• Most have 2 tagmata: cephalothorax + abdomen
• 2 pairs of antennae (only arthropods that have 2)
• Appendages on each body segment (variable number)
• Primarily aquatic (only subphylum that is)
• Mostly free-living, but some are sessile, commensal or parasitic

ex: Barnacles, daphnia, lobster

2 classes:

Hexapoda

Hexa (six) + poda (legs)

Entognatha

• Small class
• Wingless
• Base of mouthparts enclosed
• ex: Springtails

Hexapoda

• 3 tagmata: head + thorax + abdomen
• Appendages are on the head and thorax
• Mainly insects

Insecta

• Enormous group
• Usually 2 pairs of wings on thorax
• Base of mouthparts visible
• ex: Butterflies, bees, flies, beetles, etc.

Versatile exoskeleton

• Exoskeleton = external skeleton
• Called cuticle in Ecdysozoans (arthropods and nematodes)
• Secreted by underlying epidermis
• Contains chitin
• Nitrogenous polysaccharide
• Very tough material
• Found in arthropods, molluscs (radula, cephalopod internal shell), annelids (setae), and fish scales
• Heavy → limits body size
• Hard and waterproof → great protection
• Thin and flexible between segments → permits free movement of joints
• Secreted, not grown → cannot grow with the animal, thus has to be shed to allow growth

Segmentation and specialized appendages

Arthropod groups can be classified by:

• # and type of tagmata
• # of legs
• # of antennae

→ See table in Subphylums

Segmentation and specialized appendages

Metamerism

• Serial segmentation
• Segments often combined or fused into functional groups (tagmata)
• ex: head, thorax, cephalothorax, abdomen, trunk
• Appendages are often differentiated (specialized for walking, swimming, flying, eating, etc.)

Air piped directly to cells

Air piped directly to cells

→ Open circulatory system

Most land arthropods: use system of tracheae (air tubes) for gas exchange

• Hemolymph does not carry oxygen in these species
• Tracheae branch to more and more narrow tubes
• They deliver oxygen directly to tissues and cells through holes or valves

Aquatic arthropods: breathe mainly through gills, and oxygen is carried by hemolymph

Highly developed sense organs

Highly developed sensory organs

• Compound eye
• Many have antennae
• Keenly alert to environmental stimuli (touch, smell, hearing, balance, chemical reception)

Complex behaviour patterns

Complex behaviour patterns

• Simple behaviours (ex: moth going towards light) and complex behaviours (ex: female potter wasp)
• Most behaviour is innate (unlearned), but many arthropods also demonstrate learned behaviours (ex: female potter wasp must remember/learn where she left her pots)
• Social insects (ex: bees, termites) are capable of most basic forms of learning used by mammals, but not of insight learning
• Insight learning = when faced with a new problem,

can categorize memories to construct a new

response

Metamorphosis

Meta (after) + morphe (form) + osis (state of)

Types of development in insects:

Direct development

Indirect development

• Passes through larval stage capable of feeding itself
• Undergoes metamorphosis to reach adult stage

Tropic breadth through metamorphosis

• Sharp change in form during postembryonic development
• ex: tadpole to frog, larval insect to adult
• Still the same individual (development, not asexual reproduction)
• Occurs in any species with indirect development (i.e. a larval stage)
• ex: most arthropods, cnidarians, molluscs, amphibians
• Larval and adult forms live in different niches (have different sources of food, experience different selective pressures)

Asteroidea

Sea stars

• Central disc that merges with tapering arms
• Pentaradial symmetry: typically has 5 arms

Oral surface

• Near mouth
• Underside of body
• Ambulacral groove
• Runs along oral surface of each arm
• Tube feet found along them

Aboral surface

• Opposite of mouth
• Madreporite (structure where water enters

water-vascular system)

Echinoidea

Body symmetry

Lack arms, but still have typical pentamerous plan of echinoderms

Regular

• Most living species
• Radial symmetry
• ex: mouth and anus on sea urchin are on the oral and aboral sides respectively

Irregular

• Sand dollars and sea urchins
• Radial symmetry + secondary bilateral symmetry
• ex: periproct (anus) and mouth on heart urchin are at anterior and posterior ends

Sea urchins, sand dollars, heart urchins

• Dermal ossicles are now closely fitted plates that form a shell
• Spines protrude
• Long in sea urchins
• Shorter and softer in sand dollar and heart urchins

Holothuroidea

Holothuroidea

Sea cucumbers

• Elongated oral-aboral axis
• Ossicles are reduced (soft-bodied)
• Pentaradial symmetry
• Level of secondary bilateral symmetry as adults (all echinoderm larvae are bilaterally symmetrical)
• Defense mechanism: some species cast out part of their viscera (guts)
• Strong muscular contraction either ruptures the body wall or everts its contents through the anus
• Unclear why this is adaptive
• Lost parts are regenerated (but takes time and energy)