THE VARIATIONS IN SHELL STRUCTURE THAT HAVE OCCURRED IN THE PHYLUM MOLLUSCA.

The phylum is a grouping of organisms used in the classification of life. The mollusca are large and varied group of organism that includes a variety of recognizable creatures well-known for their attractive shells or seafood. The common name for mollusca is mollusc or molluscs. The molluscs are triploblastic protostomes. It has thin shell, muscular foot that is used for movement and round mantle shell (1).


These range from tiny snails, clams and abalone to the octopus, cuttlefish and squid. The snail applies to most members of the mollusca class Gastropoda that have coiled shells. These snails are of herbivorous nature, found in fresh water, marine and terrestrial environment. The snails are also grouped in to, Pulmonata group (snails with lungs) and the Praphyletic group (snails with gills form). The snails move like earthworms by alternating body reductions with stretching, with proverbially low speed. They use mucus to reduce friction and chance of being injured. The mucus helps them to prevent dangerous insects like ants (1).


Mollusc are classified into nine classes of which eight of them are still living, these classes make up the 250 000 and more species of mollusc (2).Class Pelecypoda (Bivalvia) are compacted animals with two shells hinged on the animal’s dorsal surface. This class species are found in just about every marine environment. The clams are shelled marine of freshwater molluscs belonging to the class Bivalvia. They can live for 150 years old or longer. They can be hard shelled or soft shelled, according to the degree of calcification of their shells, according to species (1).


The abalone belongs to the large class of gastropods and the Haliotidae family. The shell is rounded to oval, with two to three whorls and the last one auriform, grown in to a large ‘ear’ giving rise to the common name ‘ear-shell’. ‘The cuttlefish are the animals of the order Sepiida, and are marine cephalopods, small relatives of squids and nautilus. They are having the internal shell, large eyes, and eight arms and two tentacles furnished with denticulate suckers’ (1). The squid are the large, varied group of marine cephalopods accepted as food in cuisines as widely separated as Korean and Italian. Class Caudofoveta are small (1 – 30 mm) class of the phylum mollusca, primitive deep sea molluscs. They are worm – like molluscs that live buried head down in the floor, lacking shells or distinct muscular feet (1).

Class Monoplacophora have a single cap-like shell and they superficially resemble limpets. The class Polyplacophora, usually known as chitons, have seven or eight dorsal shell plates. Class Scaphopoda or Tusk shells have a tapered, tubular shell that is unlock at both ends (1). Class Gastropoda are the largest most successful class of molluscs. Most of the approximately 40 000 living species of this class have shells; there are few groups that have either reduced or internal shells or no shell at all. The shelled forms are generally called ‘snails’ and the forms without shells are called ‘slugs’ (1).


“The slugs and snails move by contracting and relaxing their foot muscles. They are having two types of muscles fibres that are performing different work. When they are going forward they use one set, which is pulling the snail from the front and pushing it off towards the back. While the second pair pulls the outer surface of the sole forward. Usually the sole is not divided in the snails. There are some of the snails which their sole are splited lengthwise and are known as Pomatias elegans. They can both move separately, creating a sort of pedal like locomotion” (3).


References:


1. Wikipedia contributor, Mollusca [internet]. Wikipedia, the free encyclopaedia, 2006 May 03, 08:00 UTC cited, available [http://en.wikipedia.org/wiki/Mollusca].


2. Wikipedia contributors, Phylum [internet]. Wikipedia, The Free Encyclopaedia, 2006 May 03, 12:50, UTC. Available from [http://en.wikipedia.org/wiki/Phylum].


3. Arno Brosi. 2000. The biology of snail. [Internet]. Available from [http://members.tripod.com/arnobrosi/evo.html, accessed 16/05/2006, 14:34].


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