The phylum Mollusca is the second largest group of invertebrates. There are three distinct classes in this phylum, the gastropods, bivalves, and cephalopods.
Gastropods are sometimes referred to as univalves because they appear to have one half shells that cover their bodies. The most common terms for this group are limpets, snails, and slugs. There are both fresh water and marine species of these organisms. They have a defined head, antennae, eyes and a radula which they use for feeding. The various species may be herbivores, carnivores, scavengers, detritus feeders, deposit feeders or filter feeders.
Bivalves refer to the organisms which have a symmetrical two part shell. Examples of this class include oysters, mussels, scallops and clams. Clams are filter feeders which live in the sand. They move my using an appendage referred to as the foot. Scallops also move by clapping their valves together. Oysters are sessile organisms which create homes by cementing themselves to one location – typically other oysters. Mussels are also sessile and anchor themselves to the roots of plants by special proteins called byssal threads.
Cephalopods are the largest invertebrates. This class consists of octopuses and squid. They have a highly developed nervous system and are highly intelligent. Although they do not have a hard shell, they still possess bilateral symmetry and a prominent head in place of the typical large foot. Cephalopods also have chromatophores which allow them to change colors for both communication and protection. Squid and octopuses use a form of water jet propulsion for movement. They take water into the mantle cavity and force it out of a siphon allowing them to move in the direction opposite the direction of the expulsion.
Cephalopods use their tentacles and a small bird like beak that they use to capture and consume their prey.