Mollusca

Phylum Mollusca is the second largest animal phylum after Arthropoda. They are terrestrial or aquatic (both marine and freshwater). Members of this phylum include snails, octopuses, squids, oysters etc.

Soft body of Molluscs

The word “mollusc” means “soft thing”. Molluscs have soft bodies and this explains the name of the phylum. Since their body is soft, they are fragile and find it difficult to support their body in terrestrial environment. Many molluscs solve these problems by secreting a calcareous shell, which functions as exoskeleton and prevents dehydration.

Major classes

There are five main classes of Phylum Mollusca includes Bivalves, Gastropods, Cephalopods, scaphopods (tooth shells) and Polyplacophora (Chitons). Bivalves, Gastropods and Cephalopods are commonly known mollusc animals.

Bivalves

This group includes molluscs which have a calcium carbonate shell made of two, usually similar parts called valves. These two valves are joined together with the help of a ligament at a point called hinge. Members of this class include clams, oysters, cockles, mussels, scallops etc.

Gastropods

All kinds of snails and slugs, big or small, marine or freshwater or land snails are put in class Gastropoda. This class of Molluscs has an extraordinary diversity of habitats. They are found in gardens, woodlands, deserts, mountains, lakes, small ponds, estuaries, mudflats, beaches, abyssal depths of oceans etc. They are called Gastropods because they have their feet in their ventral region.  Body of gastropods is divided into three main portions: the head, the visceral mass and the foot.

Cephalopods

Cephalopods (headfeet) are another class of molluscs which have a prominent head and a set of arms / tentacles (muscular hydrostats). These arms are modified feet. The Cephalopods are colloquially called inkfish, because of their ability to squirt ink. The Octopus, Squids and Cuttlefish are some of the common examples of Cephalopods.

Salient Notes on Molluscs

• Molluscs have a complete digestion system with mouth and anus. They also have extracellular digestion which is not found in any phylum below them in taxonomical hierarchy.
• A few molluscs have a tongue like structure that is used to scrape food. It is called Radula.
• Study of Molluscs is known as Malacology while that of Cephalopods is known as Teuthology.
• Aquatic molluscs respire through gills, while terrestrial molluscs have a primitive lung.
• Shell of Molluscs: The Molluscs, particularly bivalves and gastropods secrete an external Calcareous cell made of Calcium Carbonate. This secretion is done by mantle, a fold in their epidermis.

Ecological and Economic Importance of Molluscs

Molluscs play an important role in several food chains in ecosystems. Many marine molluscs are a part of a common human diet, such as octopuses and squids, which are very popular in Asia, and oysters and mussels, which are consumed all over the world. In addition to molluscs that are a part of the food industry, pearls made by oysters have a large commercial value.

Formation of Natural and Artificial Pearls

Pearls are made from small foreign particles that are deposited between the shell and the mantle of an oyster. These particles trigger a defense process in the organism and are gradually covered by layers of calcium carbonate secreted by the oyster, thus producing pearls.

In the artificial production of pearls, a small fragment of shell covered with mantle pieces is inserted between the shell and the mantle of an oyster and a pearl is formed around the graft.