Molluscs 2BIbDF09 10/11

What are molluscs?

They are invertebrates animals and there is an estimated number of 100'000 species in the phylum Mollusca. 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 form the group of molluscs the most important. The group of gastropods is divided by three under group: The prosobranches, the opistobranches and the pulmonés. 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.

The shell is in the shape of spiral,which is used for protection.

They have two pairs of tentacles of which the longest have eyes in the extremity.

The Cephalopods

Cephalopods differ from gastropods and bivalves. Unlike most gastropods which have shells, most cephalopods, like the octopus, miss it. Only a few cephalopods have heavy shells (Nautilus) or a small and internal shell (Squids). They are made for speed and agility; 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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