Mollusca makes the second largest phylum of non-chordate animals including snails, octopuses, sea slugs, squid, and bivalves such as clams, oysters, and mussels. This phylum contains about 100,000 described species. Among all known marine species, 23% are mollusks. But some species live in freshwater and terrestrial habitats. This group displays a broad range of morphological features such as a muscular foot, a mantle and visceral mass containing internal organs. Calcium carbonate secretes from the mantle and forms outer calciferous body shell in most of the mollusks. The size of the molluscans varies from less than one millimeter to 20 meters. They play an important role in the lives of humans because they are the source of food for many people as well as jewellery. Many molluscans are not good for human’s lives. Some cause diseases or acts as pests like the snails and slugs. Generally, the hard calciferous shells of mollusks are used to build awesome jewellery pieces. Some mollusks such as bivalves and gastropods produce valuable pearls. Natural pearls are produced when a small foreign object gets trapped in between the mollusk's body shell and mantle. Besides these, many scientists use bivalve mollusks as bioindicators of the freshwater and marine environments.
General Characteristics of Phylum Mollusca
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Classification of Phylum Mollusca
Phylum Mollusca can be divided into the following different classes:
Class-1: Pelecypoda or Bivalvia
This class comprises more than 15000 species including mussels, oysters, clams, scallops, etc. The largest living bivalve species is giant clam (Tridacna gigas) which can grow up to 47 inches (1,200 mm) in length and more than 200 kg in weight.
Examples: Mya arenaria (soft-shell clam), Mercenaria mercenaria (hard clam)
Class- 2: Gastropoda
The class Gastropoda contains more than 65,000 species which make the largest group in the phylum Mollusca comprising over 80% of all mollusks. Among 65,000 species, about 30,000 are marine, 5,000 inhabit in freshwater, and 30,000 live on land. They are the successful groups in the ocean, freshwater (ponds, streams, marshes, and lakes) and land.
Examples: Turritella cumberlandia, Diastoma insulaemaris, Epitonium charlestonensis, Pila globosa (Apple snail)
Class- 3: Cephalopoda
This class contains about 800 known living species including octopus, squid, and cuttlefish which only inhabit in marine environments. They are carnivorous and their foods consist of various fish, crustaceans and other mollusks.
Examples: Octopus abaculus, Loligo vulgaris, Sepia dubia
There are about 25 living species which all inhabit the deep sea between 175–6400 m depth ranges.
Examples: Laevipilina antarctica
They are a small group of mollusks which contains about 320 species.
Example: Neomenia yamamotoi
Class-6: Polyplacophora (Chitons)
Animals of this class are commonly known as chitons. They are exclusively marine organisms and have a worldwide distribution.
This class contains about more than 350 species. Most species live in fairly deep water within about 4,000 meters depth ranges. Among them, many species are cosmopolitan in distribution. Generally, their foods consist of small organisms such as foraminiferans and young bivalves.
Examples: Cadulus delicatulus, Dentalium americanum
Mollusca is the second largest phylum which contains over 75,000 described species. Among all known marine species, 23% are mollusks. But some species live in freshwater and terrestrial habitats.They play an important role in the lives of humans because they are the source of food for many people. The hard calciferous shells of mollusks are used to build awesome jewellery pieces. Some mollusks such as bivalves and gastropods produce valuable pearls. Besides these, many scientists use bivalve mollusks as bioindicators of the freshwater and marine environments.