Short essay on the external features of pila snail

(i) Shell:

Outer skin fold called mantle secretes a calcareous exoskeleton called shell. It is hard, globose shaped, and lemon-yellow or brownish in colour.

It is univalved (single piece) consisting of tubular whorls coiled around a central axis called the columella.

There are usually 6.5 whorls in the shell of P. globosa. Smallest and the oldest whorl lies at the apex of the shell called protoconch laid down by the larva.

Image Source: farm8.staticflickr.com

Other whorls are successively increased in size and the lowest whorl is the largest. All the whorls communicate freely; there is no partition between them.

Such a shell is called unilocular. Externally successive whorls are demarkaied by lines called sutures. On the ventral surface of the shell a mouth is situated with a smooth margin called peristome.

Its outer margin is called the outer lip while the inner one next to the collumella is the inner or columellar lip.

Collumella is a hollow rod opens through a narrow aperture, called umblicus. Such a shell bearing umblicus is called umblicated or perforated shell.

Surface of shell is marked by numerous lines of growth. The coiling of shell in pila is in right hand direction and such shells are called dextral clockwise, or right handed shell.

In the transverse section shell exhibits three distinct layers:

(a) Periostracum:

It is outer most layers composed of conchiolin.

(b) Ostracum:

It is the middle layer made up of CaCO₃.

(c) Hypostracum or nacreous layer:

It is the innermost layer and made up of plates of CaCO₃.

(ii) Operculum:

It is a door-like structure attached to the hinder part of the foot, works for the closing of mouth of the shell. It is a calcareous plate formed by cuticular secretion of the glandular cells of the animals’ foot.

Its outer surface shows many concentric rings of growth around a small sub-central nucleus. Inner surface exhibits a distinct elliptical area of creamy colour, the boss, to which the

(iii) Body:

Body is divided into four distinct regions: head, foot, mantle or pallium and viscera! Mass:

Head:

This is the anterior part of the body prolonged into a contractile snout and bears a mouth, 2 pairs of tentacles, and a pair of eyes.

First pair of tentacles arises from the sides of the head, called labial palp, they are small but highly contractile.

Second pair of tentacles arises from the dorsal surface of the head behind the snout. They are long, fleshy and contractile also.

Near the base of each tentacle projects a small stumpy eye-stalk or ommatophore, bears a small but prominent eye at its tip.

Foot:

It is large, strongly muscular ventral part of the body and triangular in shape. The grey ventral surface or sole helps the animal in creeping while the dorsal surface bears operculum. When the foot is withdrawn, operculum completely fits into the mouth of the shell and closes it.

Inside the foot are present numerous pedal glands which secrete a slimy trail during locomotion. Head complex is attached to the visceral mass by a short inconspicuous neck.

Visceral Mass:

Visceral mass is a spirally coiled collection of visceral organs. The coiling of mass occurs due to different rates of growth of visceral organs which is commonly called as torsion.

Mantle:

The skin fold which covers entire visceral mass is called mantle or pallium, a unique feature of molluscs.

Anteriorly the thick free edge of mantle serves as a protective hood for head and foot complex. When the animal is retracted. This free edge becomes distinct by the presence of parallel supra-marginal grooves.

Behind the groove is a thick band containing shell secreting glands. At the sides of the head the mantle is prolonged into highly contractile and fleshy process, the nuchal lobes or the pseudepipodia. Left one is much longer than the right and forms a respiratory siphon during aerial breathing.