Pila : An Apple Snail

Systematic position of Pila

Phylum : Mollusca

Class : Gastropoda

Subclass : Prosobranchia

Order : Mesogastropoda

Family : Pilidae

Genus : Pila

Species : globosa

Habit and Habitat

• Pila globosa is a common freshwater apple snail found in ponds, pools, tanks, lakes, marshes, paddy fields and sometimes even in streams and rivers of Northern India except Punjab.
• Pila is adapted to lead an amphibious life, it is provided with a pulmonary sac for aerial respiration and a gill or ctenidium for aquatic respiration.
• Snails may tide over long periods of droughts by remaining torpoid with the shell aperture tightly closed. It is then said to be in summer sleep or aestivation.

External morphology of Pila

Shell

• Pila is devoid of an internal skeleton; instead, it secretes an exoskeleton in the form of a shell.
• Shell forms a sort of house to live in.
• It is globose in shape.

Gross structure: Shell is an elongated structure consisting of tubular whorls coiled around a central axis called the columella.

• Various whorls lie in different planes so that the shell looks like a conical spire.
• Internally, all the whorls communicate freely with one another, there being no separating partitions between them; such a shell is called unilocular. Externally, successive whorls are demarcated by lines, called sutures.
• Body whorl opens into the exterior by a wide opening, the aperture or mouth, which is situated on the ventral surface of the shell.
• Smooth and continuous margin of the aperture is called the peristome.
• Its (peristome) outer margin is called the outer lip while the inner one next to the columella is the inner or columellar lip.
• Columella is a hollow twisted and rod-like structure, which opens to the exterior through a narrow aperture, the umbilicus, situated near the columellar lip at the end opposite to the apex of the shell.
• Surface of the shell is marked by numerous, somewhat raised lines of growth running parallel to the margin to the aperture.
• When the shell of Pila is held with the apex upward and the aperture facing the observer, it displays a clockwise coiling from the apex to the aperture so that the latter comes to lie on the right side. Such a shell is called dextral, clockwise or right-handed and is of normal occurrence in Pila and a big majority of gastropods.
  In a few species, the shell displays anti-clockwise coiling and is called sinistral, anti-clockwise or left-handed.

Torsion in Pila / Torsion in Gastropods

• Torsion is a 180^o counter clockwise rotation, or twisting, of the visceral mass, shell, mantle, and mantle cavity with respect to the head and foot.
• It is a developmental event that occurs in the larva, not in the adult.
• The head and foot remain unaltered by this rotation.
• Structures such as nerves and gut that pass between the head, foot and the visceral mass are twisted.
• Torsion is a defining characteristic of the gastropods and it is a developmental event that continues to occur in the life cycle of each individual gastropod.
• Prior to torsion, the mantle cavity and its organs are posterior. After torsion, the mantle cavity and organs are anterior immediately behind and above the head.
  Internally, the digestive tract is looped into another U.
  The stomach is now posterior and dorsal whereas the mouth and anus are anterior and ventral.
  The chemical analyser and gill comes in front.

Mantle cavity and Pallial complex of Pila

• In the anterior part there is a large space between the mantle and the body, this is the mantle cavity (or pallial cavity) (which has been shifted to the front by a process of torsion).
  It encloses a number of organs and the head can be withdrawn into it. The mantle or pallial cavity encloses within it a number of important organs which are collectively known as pallial complex.

• On the floor of the mantle cavity (pallial cavity), runs a prominent ridge, the epitaenia, which extends up to the extreme posterior end of the cavity.
  It divides the mantle cavity into two chambers – the right branchial chamber and the left pulmonary chamber. These two chambers play an important role in respiration.

[A] Organs of Branchial Chamber

1. Ctenidium or gill – It lies at the extreme right side of the mantle cavity hanging vertically downwards from its dorso-lateral wall.

• Being monopectinate (comb-like), its gill filaments or lamellae hang freely in the branchial chamber.

2. Rectum – Rectum lies to the left of the ctenidium on the floor of the branchial chamber.

• It is a raised tube-like organ, extending from the extreme posterior end ff the mantle cavity and terminating a little behind the right nuchal lobe.
• Its external opening is the anus which is surrounded by a rosette of minute papillae.

3. Genital opening – Male or female genital duct lies close to the rectum.

• In male, the penis, which is a copulatory organ, arises from the mantle edge in front of the genital opening.

4. Hypobranchial gland – It is a glandular thickening at the base of the penis. Its secretion helps in the act of copulation.

5. Renal opening

[B] Organs of Pulmonary Chamber

1. Pulmonary sac – A bag-like large organ hangs downwards from the roof of the mantle cavity, occupying a larger area of the pulmonary chamber. This organ is the pulmonary sac or lung.

• It communicates with the pulmonary chamber through an elongated opening. It helps in aerial respiration.

2. Osphradium – Arising from the mantle, adjacent to the left nuchal lobe and situated to the left side of the pulmonary chamber is a gill-like osphradium.

• It is bipectinate (feather-like) and consists of 22 to 28 fleshy and somewhat triangular leaflets.
• It helps in testing the physical and chemical qualities of the entering water and aids in the selection of food.