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Cnidarians and Mollusks
Transcript of Cnidarians and Mollusks
Cnidarians (ny DEHR ee unz) are
that have stinging cells and take food into a central body cavity. Cnidarians use sting cells to capture food and defend themselves.
with soft unsegmented bodies that are often protected by a hard outer shell. Mollusks also have a thin layer of tissue called a mantle that covers its internal organs and its main organ called a foot.
The Three Mollusk Groups
Gastropods are the largest group of mollusks. Gastropods have a single external shell or no shell at all.
Bivalves are mollusks that have 2 shells held together by hinges and strong muscles.
Cephalopods are ocean dwelling mollusks whose foot is adapted to form tentacles around its mouth. Not all have a shell. Cephalopods are the only mollusk with a closed circulatory system. They have the most complex nervous system of the invertebrates.
How Cnidarians Reproduce
How They Eat
Cnidarians reproduce both sexually and asexually. A medusa creates a polyp while a polyp creates a medusa. Some are just male or just female, while some are both genders.
Cnidarians: Both polyps and medusas obtain food in the same way, they use stinging cells to catch the animals they eat. When the stinging cell touches prey, this threadlike structure with many sharp spines explode into the food. Then the tentacles pull food into the mouth where it then passes into a hollow central body cavity to be digested.
Gastropods: Some gastropods are herbivores, some are scavengers, and some are carnivores. Gastropods use a radula ( flexible ribbon of tiny teeth ) to rip into the food.
Bivalves: Bivalves are omnivores, and are filter feeders that strain tiny organisms from water.
Cephalopods are carnivores. They capture prey using their tentacles. Then, it crushes its prey in a beak and scraped and cuts the flesh with its radula.
Cnidarians have two body plans. Both have radial symmetry, a central hollow cavity, and tentacles with stinging cells.
Polyp- Polyps are vase shaped with their mouth opened at the top. Tentacles spread around the mouth, and the polyps are adapted for life attached to an underwater surface.
Medusa- Medusas are bowl shaped and adapted for swimming. Their mouth is open downward, and the tentacles trail down. * Some cnidarians go through a polyp and medusa stage while some cnidarians are either a polyp or medusa for life.
Polyp (sea anemone)
Mollusks have bilateral symmetry and a digestive system with two openings. The internal organs are in one area.
Mollusks have an open circulatory system where blood is not always in blood vessels. The heart pumps blood into a short vessel that opens into the body spaces with organs. Blood eventually returns to the heart.
How Mollusks Breathe
Most mollusks that live in water have gills. The gills remove the oxygen from the water and the oxygen goes into the blood. CO2 goes back into the water.
Why Are There Pearls?
Only bivalves can make pearls. When an irritating object such as a grain of sand gets between the shell and mantle, the mantle produces a smooth, pearly coat to cover the object. This doesn't always create the pearls we know.
Some cnidarians have muscle-like tissues that allow them to move in different ways. Jellyfish swim through the water and sea anemones stretch out, shrink down, bend slowly from side to side, and often move slowly from place to place. A cnidarian's movements are directed by nerve cells. The cells help cnidarian respond quickly to danger and to nearby food.
A gastropod usually moves by creeping along on a broad foot. The foot may ooze a carpet of slippery mucus (mucus is basically snot for us) which makes it easier for it to move.
Larvae (baby bivalve) of most bivalves float or swim through the water. Adults stay in one place or use their foot to move very slowly. Clams use their foot to dig down into the sand.
Cephalopods swim by squeezing a current of water out of the mantle cavity and through a tube. They shoot off in the opposite direction.
Cnidarians live under water.
Gastropods live nearly everywhere on Earth such as in oceans, on rocky shores, in fresh water, and on land.
Bivalves live in all kinds of watery environments.
Cephalopods live in the ocean.