VARIATIONS IN SHELL STRUCTURE IN THE PHYLUM MOLLUSCA
To understand this better we look first at the anatomy of the molluscs. The posses a foot for locomotion, a shell, a mantle made of thin layers of body tissue covering the internal organs and an internal cavity covering the central part of the body from which most species have gills for extracting oxygen found in water. The shell is excreted by the upper surface of the mantle. The limpets (flattened, cone-shaped shelled marine molluscs) 2 produce the shell at the edge of the mantle at equal rates there by producing flat shells while other molluscs the front side secretes faster than the rear end producing a flat spiral6. Cowries are marine molluscs of the Genus Cypraea, and produce its shell by having maximum secretions along the mantle edges giving rise to a shell that look like a clenched fist. The shell is smooth and egg-shaped, with a long, narrow, “slit-like opening (aperture)” 3, 6. Some molluscs have one shell like the limpets or two (bivalves) like the mussels and oysters6.
Others like the chitons have shell plates. The dorsally carried calcareous armour shell of the chitons is protective and made of aragonite. They have diverse colours, patterned, smooth or sculptured. The shell is partitioned into eight articulating plates (valves) ‘embedded in the tough muscular girdle that surrounds the chiton's body’6. This formation helps chitons roll forming a ball when dislodged for protection and cling tightly to even irregular surfaces.4
The cuttlefish have their shells reduced and have formed an internal structure called the cuttlebone, which is porous to provide the cuttlefish with buoyancy. 6, 5 The octopus and the squid don’t have any shell but some species secrete a thin layer replica of the nautilus shell, the chambers of which are used to lays its eggs. The only hard part of the squid is the beak made of material known as chitin which it uses to get food6, 7.
In general all the molluscs that have shells use them for protection as they can roll into them and others go inside and hide into them. The chitons shell has colours that assist it in camouflage in the sea away from the predators. Others use the shell for buoyancy to help in the movements in water. Others like the squid have the shell modified to become a beak which they use to hunt their food with7.
1. Wikipedia contributors. Mollusca [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 3, 12:15 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Mollusca&oldid=51355000.
2. Wikipedia contributors. Limpet [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 4, 11:18 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Limpet&oldid=51510047.
3. Wikipedia contributors. Cowry [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 22, 10:53 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Cowry&oldid=49578379.
4. Wikipedia contributors. Chiton [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 1, 21:55 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Chiton&oldid=5
5. Wikipedia contributors. Cuttlefish [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 27, 13:49 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Cuttlefish&oldid=50421114.
6. Knight, R. BCB Biodiversity chapter 2 Building Bodies: Invertebrates Of The Oceans.
(Cited 2006 May 4) http://planet.uwc.ac.za/nisl/biodiversity/Chapter2/page_18.htm
7. Wikipedia contributors. Squid [Internet]. Wikipedia, The Free Encyclopedia; 2006 May 4, 17:46 UTC [cited 2006 May 4]. Available from: http://en.wikipedia.org/w/index.php?title=Squid&oldid=53350566.
VINCENT MUCHAI WAIRIMU
Biodiversity and Conservation Biology
University of the
Private Bag X17 Bellville