Chapter 10- Kingdom Anamalia

Phylum Mollusca (Shelled Animals)

Mollusca is the second largest phylum of the animal kingdom, and include Slugs, Clams, Scallops and Squids.

MAIN CHARACTERS

HABIT AND HABITAT

The majority of molluscs are marine. Some snails and clams inhabit fresh water while slugs are terrestrial.

NATURE

• Molluscs are primitively bilaterally symmetrical animals.

• Some molluscs serve as intermediate host of helminthes parasites and some are destructive to wooden bottom of ship.

EXTERNAL FEATURES

• The body is soft, unsegmented and consists of head, foot, mantle and visceral mass.

• The body is clothed with a one layered epidermis.

• Body is commonly protected by an exoskeletal calcerous shell of one or more pieces, secreted by Mantle.

• Head is distinct, bearing the mouth and provided with eyes, tentacles and other sense organs.

INTERNAL FEATURES

• Visceral mass contains the organs of the body in compact form.

• Body cavity is hoemocoel.

• Digestive system-tract is simple.

• Circulatory system is open.

• The mouth in many species possesses a radulla, (a tongue like structure) that can be protruded to scrape, tear or pull food.

• The respiration is by means of gills or lungs.

• The nervous system consists of cerebral ganglion, a pair of pedal cords to the foot and a pair of visceral mass.

REPRODUCTION

• Molluscs may have separate sexes or they may be hermaphrodite.

• The fertilized eggs give rise to a larva stage which transform into adult.

• Three types of larva trochophore, veliger and glochidium occur in molluscs.

CLASSIFICATION OF MOLLUSCA

The phylum Mollusca is divided into six classes:

1. Amphineura

2. Scaphopoda

3. Gastropoda

4. Bivalvia

5. Cephalopoda

6. Monoplacophora

CLASS GASTROPODA

EXAMPLE

This is the largest class of mollusca which included Snails, Whelks, Conchs, Limpets, Cowries and Slugs.

CHARACTERS

• The gastropods are mostly marine, though some live in fresh water and a few are terrestrial.

• The gastropods are asymmetrical.

• They have a well developed head and a broad muscular foot.

• Both are on the ventral side of their body.

• Their visceral organs are located on the dorsal side and are enclosed in a one piece shell which is spirally coiled.

• The gastropods use the redula to scarp food particles or to drill holes in the shells of bivalves.

• Some are carnivorous, a few are filter feeders.

CLASS BIVALVIA

• The class Bivalvia is the second largest class of the Phylum Mollusca.

• The bivalves are bilaterally symmetrical with a laterally compressed body enclosed in a shell of two pieces (valves) hinged together.
• The shell can be opened or closed.

• By closing the shell tightly, the animals can protect it self off from unfavourable environment or saves itself from predators.
• The head is rudimentary and the radula is absent.

• The foot is ventral, laterally compressed, usually wedge shaped.

EXAMPLE

Bivalves include the Clams, Mussels, Oysters and Scallops. They are mostly marine some live in fresh water but none is terrestrial.

PEARL FORMATION

• When a foreign particle living or dead enters a bivalve it causes irritation.

• The epithelial cells of the mantle start depositing concentric layers of calcerous material around it, which ultimately forms a pearl.

• Pearl culture has been successfully carried out by artificially introducing fragments of man made hard material in pearl oysters.
• Pearls are formed both in marine as well as fresh water species.

CLASS CEPHALOPODA

• The cephalopods are all marine and exhibit a high level of development.

• Nautilus is the only living cephaloped that possesses a well developed external shell which is coiled and divided by transverse Septa in chambers.

• The shell is reduced and overgrown by mantle in Squids and Cuttle fish.

• It is completely absent in Octopus.

EXAMPLES

Nautrilords, Squids, Cuttle fish, Octopus etc.

CLASS MONOPLACOPHORA

• They are primitive molluscs with a long fossil record.

• They have only one living representative, Neoporlina, which retains the segmented characteristics of annelids, lost in all other molluscs.

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