Wednesday, May 10, 2006


Phylum mollusca are the largest one in all phyla, in both size of species and in number of species which have been described. The structure of the molluscar shell is complex. It consists of outer covering and inner layers. The outer covering consists of periostracum which composed of organic chemicals and inner layers consist of calcium carbonate. The periostracum is chemically distinct and is usually referred as conchin or conchion. These provide the protection to underlying calcareous shell layers against erosion. The periostracums also serve to form microscopic shell sculpted.

The calcareous shell structures are complex and variable. The calcium carbonate crystals are laid down in an organic matrix, deposited as calcite or aragonite. Calcite is more stable, it is the main shell substance in oysters. Aragonite is the major component of freshwater and land snails.

The phylum mollusca are characterised by flexible body plan. Mollusca consist of body mass, covered with protective shell which extends head and foot. Two anatomical structures are unique to phylum mollusca, i.e. Mantle and the Redula. Mantle is a flat from visceral hump. Redula resembles a toothed tongue. Molluscar body plan is bilaterally symmetrical, and unsegmented.

The body is divided into six classes which include: monoplacophora, Polyplacopora (Amphibians, Chi tons), Gastropods (snails), pelecypoda (Bilvalva, Claims. Cephalopods (squids and octopus), and Scaphopoda (tooth shells).The Pelecypods, Gastropods and Cephalopods are numerically dominant classes. Now we can focus on members of each of these classes to get their similarities and differences. We will also examine the Molluscan locomotion and feeding.

The Gastropods are more similar to the ancestral molluscan form than any other molluscan classes. They differ from primitive ancestor, in having enlarged head and visceral mass. Gastropods are marine, they are numerous forms in both fresh water and terrestrial environment. The foot is basically broad flat organ. Gastropods are the most successful group of Molluscs not only in terms of number of species but also in wide range of habitat in which they may be found. They have well developed head that includes tentacles, eyes and various chemoreceptor as well as mouth. Gastropods foot is flattened to a creeping sole. Locomotion in gastropods is composed of muscular contraction of foot aided by mucus secretion.

Clams, mussels and scallops are the members of a class Palecypoda. Bivalves don’t have much common with snails or the primitive molluscan form expect their protective shell. Bivalves are found in the every marine environment. Most Bivalves are suspension feeders, filtering small organism and organic particles from the water. Bivalves are generally sedentary. The foot, visceral mass, and the mantle cavity dominate the body and the head is suppressed.

Bivalves have developed from primitive molluscan form. The mantle cavity has been greatly enlarged, and the gills serve as a food sorting organ and respiration. The gills also serve as brood chamber for developing young. Bivalves do no appear much in common with snails. Bivalves are sedentary. The foot, visceral mass and mantle cavity dominate the body and the head is suppressed. Bivalves have lost Radula and the majority are ciliary’s feeders with plate like food.

It includes the species which are largest known invertebrates, the most intelligent, and the fastest swimming aquatic invertebrates. They are forms with external shells and internal hard shell cephalopods are most invertebrates, and the relationship to mollusc is not obvious. The head and foot of cephalopods has become fused to form cephalised anterior end and there is reduction and loss of shell.

It is found throughout the oceans and its habitat are exclusively marine saltwater. Aplacophora are small, cylindrical, warm like and less than 5cm long. It ranges from 1mm to3cm. Like other mollusc it has no outer shell. All aplacophora have simple mantle cavity

It is found in the very deep water. It is very thin and fragile. The shell is about 1,0to 3, 5 cmlong and the members of this class have a single, cap shaped shell similar to a limpet. The shell is small ranging in size from 3 to 30mm in diameter.

The shell is composed of eight overlapping plates and these are jointed to each other on the outer margin and underside to the girdles a thickened part of the mantle. The gridles consists of scales and spines. The plates used to allow flexibility.

The wider end of the shell was head and foot. Its shell is usually four layers which are used for identification. It has a short head, conical projection with a mouth. The shell surrounds as large mantle cavity. It has no ctenidia and gas exchange is through the mantle surface. The shell is open at both ends. The shell is curved, tubular and shaped like an elephant tusk. Its average is 3 to 6cm long but range from 4mm to 15cm.


Mulcrone, S.R, 2005.Animal Diversity. Web. [Online] Available:
[2005 May 08, 10:17]

Wikipedia Contributors.Mollusca [Internet].Wikipedia, The free Encyclopedia; Available: [2006 May 05]

Linette Netshiheni
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