Taxonomy of Shellfish

10.1. General characters of Phylum Mollusca

Unit 10 - Taxonomy of Phylum Mollusca

10.1.General characters of Phylum Mollusca
The name Mollusca (in Latin mollis = soft), was first used by the French Zoologist, Cuvier in 1798 to describe squids and cuttlefish, animals whose shell is reduced and internal or entirely absent. It was only later that the true affinities between squids and multiform and other molluscs such as snails and bivalves were fully recongnised. Next to arthopods from biodiversity perspective, molluscs comprise the largest group of species. Over 160,000 species have been described, of which around 128,000 are living and about 35,000 have been recorded as fossil species. Molluscs range from forms that are only a few millimeters in diameter to the largest of all invertebrates many meters long. Like arthopods, molluscs are also found in all habitats. In the sea they occur from the deepest ocean trenches to the intertidal zone of inshore area. They are also found in estuaries, back waters, lagoons and rivers as well as on land. Molluscs occupy a wide range of habitats.

General characters

1. The body soft and bears no segmented appendages.

2. Body divided into three regions,

(i) Head (which may be indistinct in pelecypods).

(ii) Ventral, muscular foot (used for creeping, swimming) or even adhesion (arms of cephalopods).

(iii) Visceral mass

3. Bilaterally symmetrical; viscera and shell coiled in gastropods and in some cephalopodas.

4. Body triploblastic made up of 3 germ layers.

5. Body skin single layered, mostly ciliated and with mucous glands.

6. Body usually small and soft, enclosed in an envelope, the mantle that secretes shell of various shapes and sizes.

7. Shell if present, usually univalve or bivalve, constituting an exoskeleton, internal in some species.

8. A visceral mass contains internal organs, including the digestive tract, paired kidneys and reproductive organs. Visceral mass surrounded by a tegument or mantle, the outer edge of which may secrete a shell.

9. A mantle that surrounds but does not cover entirely the visceral mass and secretes a shell (if present). The mantle also contributes to formation of gills or lungs. The mantle forms a fold delimiting a pallial or mantle cavity which is flooded by water entering from outside. The gills are housed in the mantle cavity.

10. A head/foot region containing sensory organs and a muscular structure (foot) used for locomotion. The foot a muscular structure used for locomotion, attachment to a substrate, food capture or a combination of functions.

11. In many animals radula or a rasping organ present, the radula appears in the buccal cavity, bears many rows of teeth, used for grazing on food. It tears up food.

12. Coelom reduced and represented mainly by pericardial cavity, gonadial cavity and kidney.

13. The alimentary canal either straight or coiled. Jaws present in many, especially in gastropods and cephalopods.

14. Respiration by general body surface or mantle or by one to many gills or a lung in the mantle cavity.

15. Sexes usually separate (some hermaphrodite and a few protandric); gonads 2 or 1 with ducts; fertilization external or internal.

The phylum Mollusca is divided into six classes. The most important class of living molluscs is the Gastropoda comprising more than 80% of all living mollusc species.