Molluscs 2BIbDF09 10/11

What are molluscs?

They are multicellular animals with a bilateral symmetry and an exoskeleton called a shell. There are three different groups of molluscs; the Gastropods, the Cephalopods and the Bivalves. All of them have a body separated in three parts. First part is the foot is the strong, muscular portion used for locomotion. Second one the visceral mass is the soft-bodeid portion that contains internal organs and the third one the mantle is a membraous, or sometimes muscular , covering that envelops the visceral mass

Molluscs are soft-bodies animals, but most are are protected by a hard shell. Many molluscs feed by using a straplike rasping organ called a radula to scrape up food. (cf picture from Essential Biology) All of molluscs have a similar body plan.

what is a shell? It is an exoskeleton?

The Gastropods

The Gastropods include the snails and the slugs. Some of them live into the water. These molluscs can be unisexual or hermaphrodite. The Gastropods meaning stomach-footed. They move by muscle contractions, thanks their ventrally flattened foot. They can have a shell like the snails.

The Cephalopods

Cephalopods generally differ from gastropods and sedentary bivalves in being built for speed and agility. A few cephalopods have large, heavy shells, but in most the shell is small and internal (as in squids) or missing altogether (as in octopuses). Cephalopods have large brains and sophisticated sense organs, which contribute to the success of these animals as mobile predators. Cephalopods use beak-like jaws and a radula to crush or rip prey apart. The cephalopod mouth is at the base of the foot, which is drawn out into several long tentacles for catching and holding prey.

The Bivalves

Bivalves, including numerous species of clams, oysters, mussels and scallops, have shells divided into two halves hinged together. Most bivalves are sedentary, living in sand or mud in marine and freshwater environments. They use their muscular foot for digging and anchoring.

References

• http://www.earthlife.net/inverts/mollusca.html

Essential Biology with Physiology Campbell/Reece/Simon Second Edition

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