Phylum Mollusca: The animals belonging to the phylum Mollusca have soft-bodies, triploblastic and bilaterally symmetrical and coelomate. The study of Mollusca is called Malacology. They are sluggish invertebrates, with a thin fleshy envelope or mantle covering the visceral organs. The term Mollusca was derived from the term given by Aristotle to cuttlefish. Mollusc means soft. These organisms are found in the terrestrial as well as in deep seas. Their size ranges from microscopic organisms to organisms 20 metres long.

They play a very important role in the lives of humans. They are a source of jewellery as well as food. Natural pearls are formed within these molluscs.

• Insects, arachnids and crustaceans are members of the largest category of creatures on the planet: arthropods.
• Arthropods have hard, external shells called “exoskeletons,” segmented bodies and jointed legs.
• Some familiar examples are prawns, butterflies, houseflies, spiders, scorpions and crabs and some
• They exhibit organ-system level of organisation.
• They are bilaterally symmetrical, triploblastic, segmented and coelomate The coelomic cavity is blood-filled.
• The body of arthropods is covered by chitinous The body consists of head, thorax and abdomen.
• There is an open circulatory system, and so the blood does not flow in well defined blood vessels.
• Respiratory organs are gills, book gills, book lungs or tracheal system.
• Sensory organs like antennae, eyes (compound and simple), statocysts or balance organs are present.
• Excretion takes place through malpighian tubules.
• They are mostly dioecious.
• Fertilisation is usually internal.
• They are mostly oviparous.
• Development may be direct or indirect.

Body Structure of Phylum Mollusca

The body consists of a head, muscular foot, and visceral hump.

• Head: It is also known as the cephalic region. The Head consists of a mouth and sensory tentacles. The mouth has a file-like rasping organ for called radula. Radula helps in feeding. 
• Muscular foot: With the help of statocysts, it helps in the movement and maintaining a balance of these species 
• Visceral hump: It is present below the mantle, which is a soft and spongy layer of skin. The mantle is composed of epithelial cells for the protection of molluscs and muscle cells which sometimes help the molluscs to move using the force of the water. The space between the mantle and the hump is called the mantle cavity which has feather-like gills. These have respiratory and excretory functions.

Classification of Phylum Mollusca

This phylum is classified into 7 classes, namely, Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda.

• Bilateral and cylindrical body shape.
• Primarily found in benthic marine habitats.  
• The calcareous shell is absent. 
• The epidermis has spicules covering the body. 
• They lack head, shell, mantel, nephridia, and foot.

Monoplacophora

• They have a single cap-like shell enclosing the body. 
• They have a looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads. 
• Gills are externally located. 
• The Head lacks eyes and tentacles.

Polyplacophora

• Commonly known as Chitons. 
• Have armour-like 8-plated dorsal shell. 
• Bear a flat ventral foot that is adapted for suction to rocks and other substrates. 
• The mantle extends beyond the shell in the form of a girdle. 
• Have a well-developed radula.

Bivalvia or Pelecypoda

• Found in marine and freshwater environments. 
• Bilaterally symmetry and laterally compressed body enclosed by a shell in two hinged parts. 
• Most of them are filter feeders and have no head or radula.
• Gills get evolved into Ctenidia, which is a specialized organ for feeding and breathing. 
• Usually, bury themselves in sediments on the seabed. 
• Examples: clams, oysters, mussels, scallops, etc. 

Gastropoda

• Also known as Stomach foot.
• These include shell-bearing species as well as species with reduced shells. 
• Asymmetric and usually have a coiled shell. 
• The Head has a mouth, eyes, and tentacles. 
• Well-developed radula for digestion is present.
• They have modified feet used for crawling. At the centre of the foot around the perpendicular axis, visceral masses in shelled species show torsion and this is one of the distinguishing features of this phylum.
• Examples: snails, slugs, conchs, sea hares, sea butterflies, etc. 

Cephalopoda

• These are head-foot animals. 
• They are mostly found in the marine environment. 
• They display a variety of colourations which are used for camouflage. 
• Mostly carnivorous predators are found in this class. 
• The circulatory system in this class is closed type.
• Presence of a well-developed nervous system along with eyes. 
• Locomotion is facilitated by ejecting a stream of water for propulsion (“jet” propulsion). 
• Sometimes the shell is not present at all and sometimes it is present externally or internally.
• Examples: octopuses, squids, cuttlefish, nautilus, etc. 

Scaphopoda

• Also known as tusk shell or tooth shell. 
• Absence of eyes. 
• These are usually buried in the sand with the anterior opening exposed to water.
• The radula is present.
• The foot is modified into tentacles with a bulbous end, known as Captaculae, which are used to catch and manipulate prey.

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By Team Learning Mantras