Chapter 2 Mollusks, Arthropods, and Echinoderms Section 1 – Mollusks (Gastropods, Bivalves, and Cephalopods)

Characteristics of Mollusks Examples: clams, oysters, scallops, snails, squids Invertebrates with soft, unsegmented bodies that are often protected by a hard outer shell.

Characteristics of Mollusks – cont. Thin layer of tissue called a mantle that covers its internal organs Biologists classify mollusks into groups based on their physical characteristics: shell, type of shell, type of foot, type of nervous system

Body Structure Bilateral symmetry Digestive system with 2 openings Body parts are not repeated – internal organs are located together in one area

Circulatory System Most have an open Circulatory System – blood is not always inside blood vessels Blood sloshes over the organs and returns to the heart

Obtaining Oxygen Most mollusks that live in water have gills Gills have cilia and a rich supply of blood vessels Carbon dioxide, a waste gas, moves out of the blood and into the water

Snails and Slugs Gastropods are the largest group of mollusks Live in oceans, rocky shores, fresh water, on land Have a single external shell or no shell at all

Obtaining Food Some are herbivores, scavengers, carnivores Use a radula to obtain food Herbivores – sandpaper to tear through plant tissue Carnivores – drill a hole, scrapes

Movement Creeps along on a broad foot Foot oozes a carpet of slippery mucus which helps make it easier for the gastropod to move

Two-Shelled Mollusks Bivalves – are mollusks that have 2 shells held together by hinges and strong muscles Examples: oysters, clams, scallops, mussels Found in all kinds of watery environments

Obtaining Food Filter feeders that strain tiny organisms from water Capture food as water flows over their gills, food sticks to the mucus that covers the gills, cilia move the food particles into it’s mouth Most bivalves are omnivores

Movement Don’t move quickly Larvae – float or swim Adults – stay in 1 place or use their foot Oysters and mussels attach themselves to rocks Clams move

Protection Sand gets stuck between a bivalve’s mantle and shell – it irritates the soft mantle – mantle produces a smooth, pearly coat to cover the object Sometimes a pearl forms

Octopuses and Their Relatives Cephalopods are ocean-dwelling mollusks whose foot is adapted to form tentacles around its mouth. Examples are:octopuses, squid, nautiluses, cuttlefishes

Obtaining Food Carnivores - uses tentacles to capture the food and crush it Tentacles have suckers that receive sensations of taste and touch It doesn’t have to touch something to taste it because the suckers respond to chemical in the water – tastes before touching it

Nervous System Large eyes – excellent vision Most complex nervous system of any invertebrate Large brains – can remember and learn

Movement Swim by jet propulsion Squeeze a current of water out of the mantle cavity and through a tube Turning the tube changes the direction they’re going