THE VARIATIONS IN SHELL STRUCTURE THAT OCCURRED IN THE PHYLUM MOLLUSCA

The phylum mollusca are known to be the largest of all phyla. They provide some of the most familiar animals such as clams, mussels, squids and octopus. They are well known for they provide shells used for decoration as well as sea food. The phylum Mollusca also “includes lesser known forms such as the chitons, tusk shells, and solenogasters, among others”. (1)

They live in more or less “all parts of the world, from the deep water bodies like oceans to high up on mountains” (1). For them to stay alive; they need enough moisture to keep their body moisturised all the times. Those that live in hot dry deserts environment, they keep their bodies moist by curling up in their shell there by secreting a mucous plug and staying holed up until the next bit of moisture comes along.

The protective shells or shelters of the phylum Mollusca are made up of chemicals and nutrients. These shells or shelters are however different and quite a few groups have reduced or internal shells, or no shells at all. (1)

The aplacophora “are wormlike bilaterally symmetrical animals living at moderate, to very great depths, usually on or in soft bottoms” (2). They have no shell, but have calcareous spicules in the surface of their bodies. The bivalva or Apelecypoda does not have the shells because they live in water with ph greater than 5. It is difficult for them to develop shell because the ph value of the water. (2, 4)

The polyplacophora have the “shell consisting of eight, usually overlapping plates held together by a leathery strap” (4). They live in water and most survive by grazing algae from other hard substrates and rocks. The most distinguishing characteristics of chitons are that they have eight-piece shell.

The scaphopodia which are commonly known as tusk shells, “are bilaterally symmetrical and their elongate, tubular, tapering shells are open at both ends” (5). “The shell is usually an elongated cylindrical tube; open at both ends, and slightly curved” (5). However, some of the scaphoda shells are shaped more like a bloated cucumber. The shell is usually heavily not level and has small slits at the narrowest end.

The cephalopoda class includes octopus, squid, cuttlefish and nautilus. “Tentacles surround the head and a funnel coming from the mantle produces jet propulsion” (6). Externally, the shell of the nautilus is creamy white with broad reddish-brown stripes. Inside it is brilliant, iridescent mother-of-pearl.

The Gastropods generally “have a single-valved shell, which is usually spiralled; this brings their organs from a posterior position to an anterior position behind their head” (7). In most cases, the soft animal is able to pull back into their shells for protection.

The 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” (8).

The shell serves both protective and supportive purposes. The one feature common to all molluscs is the presence of a fleshy mantle. This is a fold or lobe of fleshy material, which secretes, modifies and lines the shell.


References

1. Wikipedia contributors. Mollusca [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 18, 21:38 UTC [cited 2006 May 23]. Available from:

http://en.wikipedia.org/w/index.php?title=Mollusca&oldid=53926554.

2. Wikipedia contributors. Aplacophora [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 18, 05:55 UTC [cited 2006 May 23]. Available from: http://en.wikipedia.org/w/index.php?title=Aplacophora&oldid=53815532.

3. Wikipedia contributors. Chiton [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 12, 14:07 UTC [cited 2006 May 23]. Available from:

http://en.wikipedia.org/w/index.php?title=Chiton&oldid=52835758.

4. Wikipedia contributors. Bivalvia [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 5, 12:59 UTC [cited 2006 May 23]. Available from:

http://en.wikipedia.org/w/index.php?title=Bivalvia&oldid=51672364.

5. Wikipedia contributors. Tusk shell [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 23, 12:22 UTC [cited 2006 May 23]. Available from:

http://en.wikipedia.org/w/index.php?title=Tusk_shell&oldid=49743574.

6. Wikipedia contributors. Cephalopod [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 21, 05:56 UTC [cited 2006 May 23]. Available from:

http://en.wikipedia.org/w/index.php?title=Cephalopod&oldid=54311631.

7. Wikipedia contributors. Gastropoda [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 20, 06:36 UTC [cited 2006 May 23]. Available from:

http://en.wikipedia.org/w/index.php?title=Gastropoda&oldid=54153795.

8. Wikipedia contributors. Monoplacophora [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 21, 01:25 UTC [cited 2006 May 23]. Available from: http://en.wikipedia.org/w/index.php?title=Monoplacophora&oldid=54281584.

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