VARIATIONS IN SHELL STRUCTURE THAT HAVE OCCURED IN THE PHYLUM MOLLUSCA

The phylum mollusca include organisms which are having internally and reduced shell, but in some of the phylum mollusca the shell is absent at all (Edwards, 1997). For examples, the phyllum mollusca include gastropoda, and cephalopoda. Under the phylum mollusca, we find different classes which are also different in their structural body form. These classes include ''polyplacophora; cardofoveata and solenogastres; gastropoda; scaphopoda; bivalvia; cephalopoda and monoplacophora'' (Edwards, 1997). The variations in shell structure of the above mentioned classes of the mollusca may be attributed to various locations or the evolution of the species themselves.

Mostly, the phylum mollusca have their shell structures which are planispiral flat coil in shape. Cephalopoda have head and foot which are fused together to form the anterior end and they also have atendency towards reduction and loss of shell. Although the majority of the mollusca have the planispiral flat coil structure, some of them have ''partially straight, partially coiled partially uncoiled shell, helically coiled and superficially coiled'' (Edwards, 1997). Along the period of evolution, some shells of the phylum mollusca started to become diverse during the cretaceous part. However, some shells of the phylum mollusca appear to be three dimensional while some appear to be smooth without the spines.

Gastropoda are logorithmically spiraled shell that is 180 degree rotation during its development. According to the paleoanthropologists and the geologists the rapid evolution of the mollusca provided them with the useful information about the fossils. This is because paleoanthropologists can easily link each class of the mollusca according to its variation in relation to their location. Usually the majority of the mollusca live above the seafloor. Therefore when the ''mollusca die, their remains sink down into the seafloor where they remain buried as a fossil'' (Wikipedia contributors, 2006).

As it has already been mentioned in the introductory paragraph that some of the classes of the mollusca lack shells. This is evidence in the ''two classes solenogastres and caudofoveata'' (Edwards, 1997). The lack of shells in the two classes can be attributed to their habitat which is soft sediments which are usually found in deep sea. While on the other side polyplacophora consists of an overlapping shell plates which can also be attributed to its habitat, rocky shore. The class bivalvia consists of two values of shell which are held together by the ligaments as opposed to the class gastropoda which consists of a spirally coiled shell.

Although the shells in the majority of the phylum mollusca are complex, there is a different ways in which the shells can be modified. Mostly, the shells composed of three things. These three things are periostracum, outer cover and the inner layers of the calcium carbonate. Edwards (1997) also says that the outer cover of the shell protects the inner calcareous layers of the shell against erosion or heavy storm. On the other hand periostratum acts as a microscopic shell structure.

In concluding paragraph, one can say different classes of the phylum mollusca possessed different shell structure. This is because all the classes of the phylum mollusca are not found in the same place, even though the majority of them are found in the sea. After dying while in the sea, the fossils of the remains of the mollusca provide paleoanthropologists with the evolutionary information. The fossils found in the sea will enable paleoanthropologists to make both the prediction and postdiction about the primitive and modern life of the species of the phylum mollusca.

References:

Edwards, L. A. 1997. The shell structure. [Online]. Available from:
http://www.arches.uga.edu/~amylyne/GSC/shellmakers.html

Wikipedia contributors. Mollusca [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 4, 13:39 UCT [cited 2006 May 6]. Available from: http://en.wikipedia.org/wiki/Mollusc

Wikipedia contributors. Mollusca [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 5, 14: 15 UCT [cited 2006]. Available from: http://en.wikipedia.org/wiki/Molluscs

Peter Muvhali
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