The molluscs are classified into six classes. The major classes are:

  • Gastropoda

These are asymmetrical and their body is covered with usually coiled one piece shell. The animal can withdraw itself into the shell. Both aquatic and terrestrial species are included in this class. The aquatic species have gills while in land forms the mantle cavity is converted into lungs.

The common examples are:

  1. Helix aspersa: it is commonly termed garden snail.
  2. Limax/ the slug
The gaint squid is the largest invertebrate animal reaching a length of 15 meters (almost 50 feet), including tentacles or arms.

 

  • Bivalvia (Pelecypoda)

This class includes bilaterally symmetrical aquatic mollusks. The body is laterally compressed and is enclosed by two pieces of shells hence the name bivalves. They respire by plate-like gills.

The common examples are:

  • Mytilus: (marine mussel). (ii)           Anodonta: (freshwater mussel).                               (iii)          Ostrea: (oyster)

Example of molluses

(iii)         Cephalopoda

The members of this class are bilaterally symmetrical with dorso – ventrally flattened body. All species are aquatic. The shell is much reduced and internal. In most cases it is absent. The animals are highly developed and active.

The common examples are:

  1. Loligo: (squid).
  2. Sepia: (cuttlefish).
  3. Octopus

Economic Importance of Mollusca

Some mollusks are indirectly harmful to man but most of them are beneficial. The harmful mollusks are slugs and shipworms. Slugs are injurious to gardens and cultivations. They not only eat leaves but also destroy plants by cutting their roots and stems. Teredo, a shipworm damages wooden parts of ships. But many mollusks are great source of food for man in many parts of world. Large quantities of clams, oysters and mussels are eaten in Fareast, Europe and America. Oysters are regarded as delicacy.

The brain of octopus is exceptionally large and complex for an invertebrate brain. It is enclosed in a shell-like case of ‘Cartilage’, and endows the octopus with highly developed capabilities to learn and remember. In laboratory, octopus can rapidly learn to associate certain symbols and can open a screw cap jar to obtain food.

Shells of freshwater mussels are used in button industry. Also shells of oysters are mixed with tar for making roads in America. Shells in certain parts of the world are also used for making ornaments. Some oysters also make valuable pearls e.g. the pearl oyster.