Mollusks have three distinct body zones. The head-foot contains both sensory and movement organs. The visceral mass includes the digestive, excretory, and reproduction organs. The mantle secretes the shell.
Mollusks are classfied according to the number of shells or foot shape. Members of the class Gastropoda ("univalves") have one shell and demonstrate torsion (a 180 degree counterclockwise twist of the gut). An example of a gastropod is a snail. Members of the class Pelecypoda have two hinged shells and a muscular, hatchet-like foot. An example of a pelecypod is a clam. Members of the class Cephalopoda have a muscular foot divided into multiple, arm-like tentacles. An example of a cephalopod is an octopus.
Mollusks are protected by a calcium carbonate shell, ink, and toxins.
Gastropods secrete a slime trail and their muscular foot contracts to slide over it. Pelecypods are mainly sessile (they don't move). Both pelecypods and cephalopods can use jet propulsion (think squid).
Feeding & Digestion
Pelecypods are mainly filter feeders. Cephalopods are active predators. Gastropods have a sharp radula for drilling through shells. Digestion can occur in a ciliated tract or intracellularly.
Most have an open circulatory system (blood is not restricted to circulating within blood vessels). Cephalopods have a closed circulatory system.
Excretion occurs in nephridia.
Mollusks use external gills for respiration. Diffusion also occurs through the moist skin of the mollusks.
Mollusks have a complex brain, nerves, and a compound eye.
Reproduction occurs sexually with separate sexes. Fertilization is external.