(i) Alimentary Canal:
It is a coiled tube extending from the mouth and terminating at the anus. It has three regions, the fore gut comprising of buccal cavity and oesophagus, the midgut including stomach and intestine and hindgut consisting of rectum.
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1. Buccal cavity:
Mouth leads into buccal cavity which is lined by cuticle and surrounded by a large, thick walled, highly muscular structure, called buccal mass.
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2. Buccal musculature:
Protractor muscles are well developed. Dorsally they include one median dorsal, three pairs of anterior dorso-laterals and two pairs of posterior dorso-lateral muscles.
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Ventrally they include three pairs of anterior muscles and a pair of long and strong latero-venteral muscles. These muscles assist in the protrusion and depression of the buccal mass.
3. Vestibule and jaws:
Anterior tubular part of buccal cavity is called vestibule. The posterior part of the vestibule is marked by a pair of jaws, connected by a thin cuticular membrane.
Wall of the vestibule contains longitudinal muscle fibers that form the sphinctor of the mouth. It regulates the opening of the mouth and operates the jaws at the time of feeding. Jaws are truncated and serrated from anterior cutting edge and bearing 2 or 3 large tooth like processes.
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4. Odontophore:
The posterior part of the buccal cavity forms, on its floor, a muscular tongue or odontophore. Odontophore is supported by two sets of cartilages
(i) A pair of more or less triangular superior cartilages at the top of the odontophore.
(ii) A pair of S-shaped lateral cartilages lying on the sides of odontophore.
5. Radula:
On the floor of buccal cavity lie a chitinous curved, ribbon-like structure called the radula or lingual ribbon, its anterior end bearing a pair of wing-like flaps lying over the odontophore, while posterior end lodged in radular sac flexed behind and below the buccal mass.
Radula itself is formed by secretion of the epithelial lining of the radular sac. Below the radula lies a delicate and elastic sub-radualr membrane.
Dorsal surface of the radula bears teeth arranged in numerous transverse rows. Each row contains seven teeth, one central (rachidium) and one lateral and two marginals on its either side giving the formula 2,1,1,1, 2.
The chain-saw movement of radula on the odontophore makes it an efficient organ for rasping of food particals.
The wear and tear of anterior part of the radula is maintained by regular addition of new material at its posterior end.
6. Oesophagus:
Dorsally and posteriorly buccal cavity leads into oesophagus. It turns to left and enters the visceral mass to open into the stomach.
7. Stomach:
Stomach is U-shaped organ which lies on the left side of the visceral mass below the pericardium.
It has two chambers a broad posterior cardiac chamber that receives the oesophagus and a narrow anterior pyloric chamber from which the intestine takes its origin.
The wall of stomach is folded and a short rounded blind pouch, the caecum arises from the outer wall of pyloric chamber. At the junction of two chambers, a duct from the digestive gland opens inside.
8. Intestine:
Behind the stomach a long and coiled intestine runs backwards into the viscera to join the rectum.
9. Rectum:
It comprises of thick walled tube which extends into the branchial chamber of the mantle cavity between the ctenidium and genital duct. It opens outside through anus.
(ii) Digestive glands
1. Salivary glands:
These glands are a pair of branching white mass lying one on either side of the posterior part of the buccal mass.
Their ducts enter the muscles of the buccal cavity. Saliva contains mucin-like substance and a carbohydrase enzyme.
2. Liver or hepatopancreas:
This is coiled brownish structure which lies along the coiled visceral mass. It has two main lobes, smaller in contact with the stomach and larger extending upto the apex of the shell.
Two separate ducts arise from two lobes which unite together to form a common duct before opening into the stomach.
Each lobe consists of a large number of small tubes, the alveoli. Alveoli are lined with three types of cells.
(i) secretory cells, secreting cellulose digesting enzyme,
(ii) resorptive cells, which digest proteins intracellular and
(iii) Lime cells, which store calcium phosphate. Semi digested food enters into these alveoli, where digestion of cellulose and proteins takes place.
3. Oesophageal pouches:
Below the salivary glands lie pair of oesophageal pouches which open by a narrow duct at the junction of the buccal cavity and oesophagus. These pouches-probably secrete digestive enzymes.
1. Buccal glands:
A pair of buccal glands lies in the roof of the buccal cavity where it joins to oesophagus. Each gland is divided by an oblique longitudinal furrow into two pads. Exact function of these glands is not known, they are probably of the nature of accessory digestive glands.
(ii) Food, feeding and Digestion:
Pila is herbivorous, feeds generally on succulent plants like, Pistta, Vallisnaria etc. Sometimes it feeds upon detritus also.
The chain-saw movement of radula helps in rasping of food particles. By the action of sphinctor and protractor muscles of the buccal mass, the two jaws move up when mouth opens and cut the leaves of plants. Thus the food is cut and masticated inside the buccal cavity.
In buccal cavity food particles mix well with saliva, which contains carbohydrase enzyme, for digestion of starch into sugar.
In the stomach the extra-cellular digestion takes place by the secretion of digestive enzymes of liver and digestive glands.
Absorption of digested food takes place chiefly in the digestive gland and the intestine. Undigested food is excreted through anus into the branchial chamber which finally goes to outside along with outgoing water current.