VARIATION IN SHELL STRUCTURE THAT OCCUR IN PHYLUM MOLUSCA

Macology, which is the study of molluscs, defines molluscs generally as soft-bodied animal that usually produces exoskeleton. This external skeleton is called a Shell. A Shell is made up of calcium carbonate (CaCO3). The shell serves both for protection and supporting functions. The one feature common to all molluscs is the presence of a fleshy mantle. This is a fold or lobe (or a pair of them) of fleshy material, which secretes, modifies and lines the shell. These animals need moisture to survive so they live in all parts of the world from deep water oceans to high upon the mountains. There are seven different classes of molluscs which are recognized.


The Gastropoda, this class is composed of 60,000 species. This is where we find species like Snail and Slugs. They have a well developed head with tentacles, often highly developed foot for crawling. Shells can only be formed in fresh waters that are non-acidic, and in the ocean at depths above the level where the water becomes under saturated with Calcium Carbonate. Gastropods show different shapes, colours, patterns and sculpturing of their shell. In some gastropods, the shell is only conspicuously coiled in the early stages when it is still young. The coiling will disappear when the species grows and when it is an adult it will develop a single whorl shell which will be largely expanded. In many other gastropods, the shell has become more reduced or it is completely absent. In other cases the foot and mantle are very large and the mantle has reflexed backwards over the shell so that it becomes totally covered. These animals are no longer able to pull their bodies completely into their shells.


The Bivalves comprise about 10,000 living species. They have two valves made of calcium carbonate. Bivalves are mollusks in which the mantle cavity has been greatly enlarged in size, and whose gills, in addition to their respiratory function, act as a food sorting organ. In some clams, the gills also serve as a brood chamber for developing young. The edges of the mantle in a clam are partly fused to form a pair of siphon that pump and circulate huge quantities of water through the mantle cavity.


The shell and mantle have become enlarged to cover the foot and mantle cavity completely. The head region has atrophied, the radula is totally absent, and the body is flattened laterally. A clam shell consists of two halves or valves usually equal in size and shape, which are connected above by a flexible ligament whose resilience keeps them slightly open at the bottom. The valves of the shell can be shut by the contraction of one or two large muscles. The spring like ligament tends to hold the shell open, while the muscles enable the clam to shut its shell. The molluscan foot has been altered into either a digging organ or a secretor of holdfast fibres, since different kinds of clams can be free-living burrowers, remain fastened to one spot during most of their life, or bore into rocks and wood.

The Scaphopoda have about 350 living species. Tooth shells are occasionally called tusk shells; they are a member of the class Scaphopoda. The shell, which looks like a miniature elephant tusk, is open at both ends. The narrow end often protrudes above the mud or sand in which the animal lives. Water currents enter a long mantle cavity that is lined with simple folds instead of gills.


A combination of ciliary actions and muscular contractions of the foot circulates water and expels waste materials from the narrow end of the shell. Both head and foot can be extended from the broad end of the shell. The foot is modified to serve both in burrowing and feeding, it is used firrt to raise or lower the animal in the substrate to the point where food is available
The Polyplacophora are often considered by scientists to be the most primitive of all existing molluscs. Strictly marine, the majority of the chiton species inhabit rocky seashore environments where their low dome-shaped shells are well suited to withstanding the violent serge of ocean waves. They have a very broad creeping foot and a flattened visceral hump. These organs are protected by an articulated shell of eight calcareous plates whose edges are formed by and embedded in a flexible part of the mantle called girdle.


The Aplacophora are about 250 described species, mostly found in deeper waters. The class Aplacophora, animals also known as Solenogasters consists mostly of small worm-like molluscs that or feed upon the cnidarians. They have no shell, but have calcareous spicules in the body surface. The foot is restricted to an anterior pedal shield or to a narrow groove running the length of the body. Aplacophorans have a radula and a posterior mantle cavity. Some are detritus feeders, others are predators.


The Cephalopoda have only about 400 species in existence now. Only the tetrabranchs produce an external shell in this class of molluscs. Externally, the shell of the nautilus is creamy white with broad reddish-brown stripes. Inside it is brilliant, iridescent mother-of-pearl. The nautaloid shell is very complex, chambered and spiraled over the head of the animal. Even though coiled, it is radically different than that of the gastropods, being divided by transverse septa creating internal chambers.


The Monoplacophora are the limpet-shaped molluscs which are segmented like worms. The internal vital organs are duplicated in each segment of this animal.
Monoplacophorans have a single, large, bilateral shell. The shell is a simple depressed limpet or disk -shaped valve, less than 25 millimetres across usually and is often thin and fragile. The outer surface of the adult is covered with a protective sheath. On the inner surface of the shell, there are significant paired muscle scars, suggesting segmentation.


Reference List


1. Wikipedia contributors. Monoplacophora [Internet]. Wikipedia, The Free Encyclopaedia; 2006 April 22, 20:03 [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Monoplacophora


2. Wikipedia contributors. Scaphopoda [Internet]. Wikipedia, The Free Encyclopaedia; 2006 May 04, 20:07 [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Scaphopoda

3. Wikipedia contributors. Caudoveata [Internet]. Wikipedia, The Free Encyclopaedia; 2006 April 9, 09:01 [cited 2006 May 4]. Available from:
http://en.wikipedia.org/wiki/Caudofoveata

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