Habit habitat and external morphology of Pila: An apple Snail.

Habit and Habitat of pila: An Apple Snail:

An apple snail is commonly found in freshwater, ponds, pools, tanks, lakes, marshes, rice field, paddy field and sometimes even in streams and rivers. It is one of the largest freshwater molluscs. It occurs abduently in that area where there is a large amount of aquatic vegetation like Vallisneria and pistia, forming the food of this snail. They are adapted to lead an amphibian life I,e. for life in both land and water.The animal creeps with its ventral muscular foot, at the traditional ‘snail pace’ which average about 5 cm a minute a full speed. The movement of the apple snail i,e Pilais like the gliding movement of planarian. During the day of rainy Apple, snail comes out of the ponds and makes long terrestrial tours, thus, respiring air directly. Snail may tide over the long period of drought in a dormant condition and buried in the mud; this period of inactivity is called aestivation or summer sleep. While moving and feeding , it does not leave shell-house but carries it on back.

External Morphology of Pila: An apple Snail:

External morphology of Pilais differentiated into Shell of Pila, and Body of Pila. A short description about this is below:

[I] Shell of Pila:

Pila is devoid of an internal skeleton; instead, it secrets an exoskeleton in the form of a shell. Shell forms a sort of house to lie in. It is globose in shape. The shell of An apple Snail as in Gastropods is univalve but coiled around a central axis a right- handed spiral.

Gross structure.

The shell of an Apple snail varies in colour from lemon yellow, brownish or even blackish in colour.Shell of Pila is an elongated structure consisting of tubular whorls coiled around a central axis called the columella. The top of the shell of an Apple Snail is the apex which is formed first and growth of shell takes place from it, the apex contains the smallest and the oldest whorl.Below the apex is a spire consisting of several successively larger whorl or body and the largest whorls or coils followed by penultimate whorl and the largest whorl or body whorl which encloses most of the body.Externally, successive whorls are called sutures.

Internally all the whorls of the shell are freely communicated with one another; such a shell is called unilocular. Body whorl opens to the exterior by a wide opening, the aperture or mouth, called peristome from which the head and the foot of the living animal can protrude. When viewed from the ventral side with the peristome facing the observer, the mouth lies to the right of the columella and the shell is spiralled clockwise, then it is spoken of as being right handed or dextral.The outer margin of the mouth is called an outer lip, and the inner margin as an inner or columellar lip. In the centre of the shell runs a vertical axis or columella around which the whorls of the shell are coiled; the columella is hollow and its opening to the exterior is also called known as an umbilicus. shell with an umbilicus is umbilicate or perforate.

Microscopic Structure of Shell:

Microscopic structure of shell of An apple snail consists of an outermost pigmented layer called periostracum made of a horny organic conchiolin, below this is a prismatic layer made up of crystalline calcareous plates running vertically, the innermost nacreous layer is made of calcareous plates running longitudinally. Shell of Gastropoda displays an infinite variety of shapes, colours, patterns and sculpturing. Inside the shell is the mantle which secretes the shell. Shell is secreted by the underlying mantle.

Operculum:

The operculum attached dorsally to the hinder part and in fact secreted by the glandular cells of the foot. It is a calcareous operculum fitting into the mouth of the shell. Its outer surface shows a number of rings of growth around a nucleus. The inner surface has an elliptical boss for attachment of muscles, the boss is cream-coloured and is surrounded by the groove.

[II] Body of Pila: An apple snail:

The body of Pila consists of four distinct regions. They are the head, foot, visceral mass and mantle.In an expanded animal the head and foot come out of the shell- mouth but the visceral mass lies inside the shell whorls. A columellar muscle arises from the foot and is inserted in the columella, it attaches the body to the shell and it has withdrawn the animal inside and closes the operculum.

• Head:

The head is the anterior fleshy part of the body overhanging the foot. The head of an apple snail is a distinct head produced into a snout, the head bears two pairs of tentacles. The first pair of tentacles or labial palps are small and lie in front, behind them there is a second pair of tentacles which are long. The tentacles are hollow and capable of much extension and contraction. Behind the tentacles, the head has a pair of eyes borne on stalk or ommatophores. Mouth is a verticle slit-like aperture, lying ventrally between the bases of the first pair of tentacles.

• Foot

The ventral part of the body which is large, strongly, muscular, form the foot. Its lower surface is grey and flattened sole. It is triangular with the apex pointing backwards, it is used for creeping. Its upper surface is spotted and the dorsal posterior surface bears the operculum. When the foot is withdrawn the operculum closes the mouth of the shell. In the foot is a pedal mucous gland which forms a slime trail during locomotion. Waves of contraction which sweep from the anterior to the posterior end of the foot provide the main power for locomotion.Head –foot complex is attached to the visceral mass by a short inconspicuous neck.

• Visceral mass:

It constitutes a sort of hump on the dorsal side, containing all the visceral organs. It is soft and grey to dark brown in colour.It is spirally coiled like the shell in which it lies, occupying the body whole as well as the side of the visceral masses away from the columella. The visceral mass exhibits the phenomenon of torsion which is distinct from coiling.

• Mantle:

The skin of the visceral mass form a thin and delicate covering called the mantle or pallium, and if forms a hood over the animal when it is withdrawn.The edge of the mantle is the thick and contains shell gland which secretes the shell, above the thickened end there is a supra-marginal groove.The mantle also has two fleshy lobes called nuchal lobes or pseudopodia which are joined on either side of the head. The left pseudopodium forms a long tubular respiratory and a respiratory current enters, through it, the right pseudopodium is less developed and not a regular tube, respiratory current passes out through it.left one is much longer than the right and forms a respiratory syphon during aerial breathing.

Reference:

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L.,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

http://www.biologydiscussion.com/

https://en.wikipedia.org/wiki/Pila_(gastropod)