Monday, May 08, 2006


The largest phyla in all phylum are called phylum mollusca. It is large in number of species and in both size of species. The structure of phylum mollusca shell is difficult. Phylum mollusca consist of two layers, which are the outer and inner layers. The inner layers have calcium carbonate and the outer layers consist of peristracum. This periostracum has organic chemicals and the chemical that occurred in the periostracum is referred as conchion. Calcareous shell is protected by this periostracum. Periostracum plays an important role in the formation of microscopic shell. The calcareous shell structure that occurred in the inner layers is very complex and variable. The calcium carbonate that occurred in the outer layer has crystals. Those crystals are laid down in an organic matrix which is deposited as a calcite. Even though the calcite is the main shell in oysters.

The body of mollusca is bilaterally symmetrical, and its body plan is flexible. Phylum mollusca consist of body mass. The structures of phylum mollusca are unique. Mantle and Redula are the structure of phylum mollusca. Redula is like a toothed tongue and the mantle is a flat. The body of phylum mollusca is divided into six classes. That is, Monoplacophora, Polyplacophora, Gastropods, Pelecypoda, Cephalopods and Scaphopoda. The dominant classes in phylum mollusca are Gastropod, Cephalopods and Pelecypods. Now we can focus on members of each of these classes to get their similarities and differences and the variation in shell structure.


Snails are good examples of this class. Gastropods are like the ancestors of mollusca than other classes. They differ from primitive ancestors, because they have enlarged head. Gastropods are marine. The foot of Gastropods are broad flat organ. Gastropods are most successful group of phylum mollusca. It is large due to the wider range of habit. The class gastropods are large, not only in terms of number of species. Gastropods have well developed head. This head includes tentacles, eyes and mouth. Its foot is fattened.


Members of this class are claims, mussels, scallops and bivalves. Bivalves do not appear much in snails. It also found in every marine environment. Most of bivalves are suspension feeders, in small organism and organic particles from water. Bivalves are sedentary. Bivalves have developed from primitive molluscan form. It has not Redula. The body of bivalves is dominated by mantle cavity and the head is suppressed. The majority of bivalves are cilliary feeder with plate-like food. Bivalves have gills which serve as a developing young and serves as a food sorting organ and respiration.


Members of this class are squids and octopus. Cephalopods are invertebrates. It consists of external shell and internal hard shell. The relationship between Cephalopods and mollusca is not obvious. The head and foot of Cephalopods are combined to form cephalised frontal end, because of these process the loss of shell and reduction will occur.


Aplacophora has no outer shell. All Aplacophora have mantle cavity. Aplacophora are found throughout the oceans. Its habitat is fully marine salt water. Aplacophora are small and cylindrical. All Aplacophora have simple mantle cavity.


Monoplacophora are mainly found in very deep water. It is very thin and fragile. Members of this class have a single, cap shaped shell which is similar to a limpet. The shell of monoplacophora is small and range from 3 to 30mm diameter.


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 from:
[2006 May 08, 10:17]

Wikipedia contributors. Coordinated Universal Time [Internet]. Wikipedia, Mollusca, The free Encyclopedia; 2006 May 05, 10:15 UCT [Cited 2006 May 08] Available from:

Linette Netshiheni
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  • Hi Linette!

    This is interesting, but there are some parts that don't flow well (for example, there's a bit of a disconnect between your opening sentence and the rest of your opening paragraph). Also, you spend too much time on topics that don't deal directly with the topic - shell structure variation. As an added point, why is it that your blog doesn't seem to be runing?


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