The below mentioned article provides notes on Mollusca (snail).
The molluscs (mollis = soft) are soft-bodied animals with a hard external or internal shell. The phylum includes forms like chitons, snails, slugs, clams, oysters, sepia, loligo, octopus, etc. Generally aquatic, some terrestrial and amphibious forms are also present.
In Mollusca A muscular foot is present as the thickening of the ventral body wall. The foot is meant for locomotion, but is modified in different forms, i.e., squids, the foot is modified as arms. The body of Mollusca is bilaterally symmetrical and this symmetry is modified in snails. Excepting tentacles in some no other appendages are present.
The internal organisation is well-developed. The body cavity is represented by a small coelom and hoemocoel. The digestive system includes an alimentary canal consisting of mouth, coiled digestive tube, anus and a liver as digestive gland.
The circulatory system is well- developed and contains heart, blood vessels and hoemocoelomic spaces. The excretory functions are carried by specialised organ called kidney. In aquatic molluscs the gills are the respiratory organs, but in terrestrial forms there are lung-like structures for respiration.
The nervous system includes well-developed ganglia, commissures and connectives. Certain sensory organs are also present. The phylum includes both monoecious and dioecious forms. The fertilization may be external or internal. Two larval stages, Trocophore and Veliger, are seen in the life-history of molluscs.
Two Common Snails:
Two snails—one aquatic and one terrestrial, are very common in our country. The aquatic one is popularly called Apple snail (Fig. 119) is scientifically known as Pila globosa and the terrestrial one, garden snail is called Achatina fulica.
In both, the soft parts are covered by a hard, right handed spiral shell. In Achatina there are eight coils and 5-6 coils are present in Pila. In the body of the Pila, the opening of the shell is guarded by a flat plate called operculum, which remains attached with the foot.
The operculum is absent Achatina. In Achatina the outer part of the shell may possess stripes of various colours. In both these forms the body consists of a distinct head, which is more prominent in Achatina.
The head contains in both, 2 pairs of tentacles and an opening called mouth. On the right side of the body in Achatina, there is an aperture called pulmonary opening which is in communication with the respiratory organs. Food of both these snails are leaves and other parts of plants.
