Molluscs 2BIbDF09 10/11

What are molluscs?

They are invertebrates animals. There are three different groups of molluscs; the Gastropods, the Cephalopods and the Bivalves. All of them have the same body plan separated in three parts. The first part is the foot which is the strong, muscular portion used for locomotion. The second one is the visceral mass, a soft-bodied portion that contains internal organs and the third one is the mantle ????is a membraous, or sometimes muscular , covering that envelops the visceral mass???? Many molluscs feed by using a rasping organ called a radula. (cf picture from Essential Biology)

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 and the others live in somewhere is humid. 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.

This shell is mostly in a spiral and it is used for protection.

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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