What makes a mollusc? All animals in the phylum Mollusca have three things in common. They have a muscular foot used for movement, a visceral mass that contains most of the organs and a mantle that extends beyond the visceral mass and secretes a shell. Also, most molluscs have a radula, which is like a conveyor belt with curved teeth that continuously thrusts outward and retracts. But each class of mollusc has its own unique characteristics.
Chitons are oval shaped snail-like animal with a shell that is divided into eight different plates. All chitons live in a marine environment that feeds on algae. It is exceedingly difficult, however, to separate a chiton from a rock by hand, thanks to its foot, which acts like a suction cup.
Gastropods make up about 75 percent of the mollusc phylum. While many gastropods are marine, a good portion of them live in fresh water, and some of them have made the transition to living on land. Snails and slugs are both members of the gastropod class. While slugs do not have a shell like most gastropods, they still have a mantle. In fact, most gastropods that have a shell has been reduced to help with movement. Also, the cone snail has become a predator which uses the teeth of its radula like poison darts.
Clams, oysters, scallops and mussels all belong to the mollusc phylum and are considered bivalves. The main characteristic for bivalves is their shell is divided into two halves and is hinged by powerful muscles along the mid dorsal line. Interestingly, bivalves have no discernible head, and their radula feeding apparatus has been lost. Instead, they are filter feeders that feed on small-food particles floating in the water. Bivalves can also move with the help of their foot, but they usually are quite slow.
Octopuses and squids belong to the mollusc phylum but have made several evolutionary adaptations that help them stand out from the rest of the phylum. They are active predators and have beak-like jaws that are used to crush their prey. Some species of octopuses are among the smartest animals in the world. The foot of a cephalopod has evolved into the animals tentacles and a siphon to push water. Most cephalopods do not have shells and are the only mollusc to have closed circulatory systems. Scientists still have not been able to determine how large some species of squid can get. In fact, the only reason scientists know these species exist is because they have been discovered in the stomachs of sperm whales.