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Mollusca: Introduction
Aravind N. A.
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The name Mollusc (=Mollusk) was derived from Latin mollus meaning soft. This term was first used by Cuvier, a French zoologist in 1798 to describe squids, cuttlefish and animals whose shell is reduced, internal or entirely absent. Molluscs are highly successful animal group in terms of ecology and adaptation and they are found in nearly all habitats ranging from deepest ocean trenches to the intertidal zone, freshwater and land where they occupy a wide range of habitats. Thus, during their evolution, they have become adapted to living in nearly all available habitats. The diversity is highest in the sea. Of the recognized eight classes, all are represented in the sea, followed by freshwater with two (Gastropoda and Bivalvia) and terrestrial has only one class (gastropoda) . Among molluscs, octopus and squid are advanced animals. Giant squid are also the largest invertebrates, weighing up to 2,000 kg. Most molluscs, however, are about 0.5 to 20 cm (about 0.2 to 8 in) long and some are hardly visible to naked eye. An ideal mollusc would have a single flat, muscular foot and the body would have a head at one end and an anus at the other. Above the body would be an external shell mounted on a visceral hump containing internal organs. However, slugs completely lack shell. Most well known molluscs include clams, oyster, snail, slug, octopus and squid of which most are high economical value. Figure 1 shows different external parts of a mollusc.

Figure 1: Showing different parts of a snail

The phylum Mollusca is divided into 8 classes with varying number of species in them; the most important class is the Gastropoda comprising more than 80% of all living mollusc species. The species belonging to this class occurs in marine, freshwater and terrestrial habitat. Where as bivalves occurs both in freshwater as well as marine but not in terrestrial habitat. Rest six classes of mollusca are exclusively marine (Table 1).

Molluscs are very abundant and form an important link in the food chains. A large number are herbivores or grazers and hence are important in nutrient recycling along with the other soil invertebrates. Numerous molluscs are important food source for humans such as clams and snails. Some gastropods are pests and damage crops and others harbor disease-causing parasites such as rat lungworm and causes Schiostomasis in humans.

Table 1: There are eight classes of molluscs

Class

Characters

Examples

# species

Habitat

Aplacophora,

Body is wormlike. No shell exists

 

320

Marine

Polyplacophora

Series of eight shell plates (valves) in a row and are well adapted to clinging on rocks.

Chitons

600

Marine

Monoplacophora

Limpet like and large flattened foot surrounded by the pallial groove, in which are situated 5 or 6 pairs of gills.

Neopilina

11

Marine

Bivalvia

Shell divided into two valves

Clams

8,000

Marine and Freshwater

Gastropoda

Body asymmetrical and have only one shell (Snails) or shell-less (Slugs)

Snails and slugs

40,000- 150,000

Marine, Freshwater and Terrestrial

Cephalopoda

Foot and shell reduced arms around the mouth

octopuses, squid, and cuttlefish

786

Marine

Scaphopoda

tubular tusk or tooth-shaped shell which is open at both ends

Dentalium

350

Marine

Rostroconchia

Extinct (status from fossils)

 
 
Note: Class Rostroconchia is extinct and represented only in fossils. This is probably ancestors of bivalves and more than 1,000 species existed.

 

Box: Key features of Mollusca

  • Body bilaterally symmetrical; unsegmented; usually with a definite head.
  • Ventral body wall specialised as a muscular foot , used chiefly for locomotion.
  • Dorsal body wall forms the mantle, which encloses the mantle cavity, is modified into gills or a lung, and secretes the shell (shell absent in some).
  • Surface epithelium usually ciliated, bearing mucous glands and sens ory nerve endings.
  • Coelom mainly limited to area around heart.
  • Complex digestive system; rasping organ (radula) usually present.
  • Open circulatory system of heart, blood vessels and sinuses.
  • Gaseous exchange by gills, lung, mantle or body su rface.
  • Sensory organs of touch, smell, taste, equilibrium, and vision (in some); eyes highly developed in cephalopods

(http://www-biol.paisley.ac.uk/Courses/Tatner/biomedia/units/moll10.htm)

 

Box: Malacology

The scientific study of molluscs is called malacology . "Malacology", comes from the Greek word for soft, “malacos” . The term "conchology" is also used for the study of molluscs, however, it is usually applied to those that study only the shells.

Bibliography of non-marine molluscs of the Western Ghats, India

Acknowledgements

Thanks to International Foundation for Science (IFS), Sweden for funding the work on impact of land use change on land snails of the Western Ghats, India.

Aravind N. A.
Ashoka Trust for Research in Ecology and the Environment (ATREE),
#659, 5 th A main, Hebbal, Bangalore 560024.
Phone: +91-80-23530069, 23638771
Fax: +91-80-23530070
Email: aravind@atree.org
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