Phylum Mollusca
மொல்லஸ்கா தொகுதி
Mollusca is the second-largest animal phylum. Do not confuse their calcareous shell with the exoskeleton of Arthropoda. One is calcium carbonate; the other is chitin. Know the difference, or lose the rank.
The idea
Molluscs are soft-bodied, triploblastic, and coelomate animals. The body is divided into a head, a muscular foot, and a visceral hump. A mantle cavity exists between the hump and the mantle, housing feather-like gills for respiration and excretion. The mouth contains a file-like rasping organ for feeding called the radula.
Why it matters
The mantle cavity and gills are adaptations for respiration in aquatic environments, while the radula is specialized for efficient feeding.
NEET lens
NEET focuses on the 'radula' and the 'mantle cavity' functions. Expect matching-type questions linking the organism to its specific feature. Memorize the presence of the calcareous shell and the visceral hump as diagnostic characters. Do not skip the examples like Pila, Pinctada, and Sepia.
Clinical significance
Molluscs act as intermediate hosts for trematode parasites like Fasciola hepatica. Humans contract infections by consuming contaminated water or aquatic vegetation. Understanding their life cycle is critical for public health and preventing liver fluke infestations.
Introduction
From the tiny garden snail to the giant squid — the largest invertebrate on Earth — Phylum Mollusca contains an astonishing range of body forms. What unites a snail, a clam, and an octopus? A soft, unsegmented body, a muscular foot, and in most cases a protective shell. Mollusca is the second largest animal phylum after Arthropoda, and NEET tests it more often than its size might suggest.
Core concept
Phylum Mollusca features: (1) soft, unsegmented body; (2) triploblastic, coelomate; (3) bilaterally symmetrical (except gastropods — asymmetrical due to torsion); (4) body divided into head-foot and visceral mass; (5) mantle — a fold of skin that may secrete a shell; (6) radula — a file-like rasping organ for feeding (absent in bivalves); (7) open circulatory system (except cephalopods — closed); (8) gills (ctenidia) for respiration. Three major classes: Gastropoda (snails, slugs — e.g. Pila, Helix); Bivalvia/Pelecypoda (clams, oysters — e.g. Unio, Pinctada); Cephalopoda (squids, octopus — e.g. Loligo, Octopus, Sepia). Pinctada (pearl oyster) produces pearls.
Go deeper
Cephalopods are the most advanced molluscs — they have a closed circulatory system and a well-developed nervous system with large eyes. The radula is a uniquely molluscan structure with no equivalent in other phyla. Gastropod torsion — a 180° rotation of the visceral mass during development — is one of the most dramatic developmental events in the animal kingdom.
From Class 10
You may have seen snails and clams as examples of molluscs. At NEET level, Phylum Mollusca is the second largest animal phylum, and you need to know its defining features, three major classes, and key examples.