VARIATION IN SHELL STRUCTURE OCCURRED IN THE PHYLUM MOLLUSCA

The mollusks constitute one of the largest phyla of animals, both in numbers of living species and in numbers of individuals. All mollusks have a soft body, which is protected by a hard, calcium- containing shell. Mollusks are triplobastic (ovum which has three primary germ layers-ectoderm (outer), mesoderm (middle) and endoderm (inner layer) protostomes. A significant characteristic of mollusks is their possession of a coelom, a fluid-filled cavity that develops within the mesoderm. The coelom functions as a hydrostatic skeleton which also provides space within which the internal organs can be suspended by the mesenteries. The body cavity is filled with blood. Mollusks have a mantle (fold of the outer lining of the shell and a muscular foot that is used for motion-movement). Many mollusks mantle produce a calcium carbonate-external shell and their gills extracts oxygen from water (1).

Phylum Mollusca are divided into different classes which have different types of shells:-

Class Gastropoda

Most of the approximately 40,000 living species of gastropods have shells, however there are quite a few groups that have either reduced or internal shells, or no shell at all. Although most Gastropods are marine, there are numerous forms in both freshwater and terrestrial environments. Most members have a shell which is in one piece and typically coiled or spiralled which usually opens on the right side (2). Some species have an operculum which operates as a trapdoor in order to close the shell.

Class Pelecypoda (Bivalvia )

They have two part shells with both parts being less or more symmetrical. The class include scallops, clams, oysters and mussels. Their shells are composed of calcium carbonate and the shells are very strong. In some bivalves the shell is made up of calcium carbonate known as aragonite which is not very strong (3).

Class Cephalopoda

This class includes species like squid, octopus and cuttlefish. Their shell is either internalized or is absent. The cuttlefish is the only modern shelled cephalopod. It occupies only the outermost portion of its elaborate and beautiful shell, the rest of which serves as a flotation chamber. In the squid and its relative, the cuttlefish, the shell has become an internal stiffening support, and the octopus does not have a shell entirely (4).

Class Monoplacophora

They have a single, flat, rounded bilateral shell which resembles chitons. Their shells are often thin and fragile and their shell apex is forward (5).

Class Polyplacophora

This class include the chitons which have seven or eight dorsal shell plates embedded in the tough muscular girdle that surrounds its body. Their calcareous armour shell that they carry dorsally acts as a protective measure. Their shell is made up of aragonite and colored (6).

Class Scaphopoda

They are also called the tusk shells because their shell is openly curved plan spiral tubular shell which opens at both sides and the shell resembles an elephant tusk. They have tapered, tubular shells that open at both ends (7).

Reference:

1.Wikipedia contributors. Council of Science Editors [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 4, 3:39 PTA [cited 2006 May 4]. Available from: http://en.wikipedia.org/wiki/Mollusc

2.Wikipedia contributors. Council of Science Editors [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 3, 20:07 PTA [cited 2006 May 4]. Available from: http://en.wikipedia.org/wiki/Gastropoda

3.Wikipedia contributors. Council of Science Editors [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 3, 19:00 PTA [cited 2006 May 4]. Available from: http://en.wikipedia.org/wiki/Bivalvia

4.Wikipedia contributors. Council of Science Editors [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 03: 04:57 PTA [cited 2006 May 4]. Available from: http://en.wikipedia.org/wiki/Cephalopoda

5.Wikipedia contributors. Council of Science Editors [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 22, 20:03 PTA [cited 2006 May 4]. Available from: http://en.wikipedia.org/wiki/Monoplacophora

6.Wikipedia contributors. Council of Science Editors [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 04, 15:32 PTA [cited 2006 May 4]. Available from: http://en.wikipedia.org/wiki/Polyplacophora

7.Wikipedia contributors. Council of Science Editors [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 23, 12:22 PTA [cited 2006 May 4]. Available from: http://en.wikipedia.org/wiki/Scaphopoda

Lethabo Mosomane
CSIR
Pretoria
0001
Tel: 27 12 841 2133
Fax: 27 12 842 3676
mail: lmosomane@csir.co.za
http://lmosomane.blogspot.com

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