DIGESTIVE SYSTEM OF PILA NOTES

DIGESTIVE SYSTEM OF PILA

It consists of

I) Alimentary canal
II) Digestive glands

Alimentary canal of Pila is a coiled tube extending from the mouth and ending at the anus.

Entire canal may be divided into three regions
A) Foregut - buccal cavity and oesophagus
B) Midgut - stomach and intestine
C) Hind gut – rectum

A) Foregut

1) Buccal cavity:
 This is a chamber into which mouth opens

 It is lined by cuticle and surrounded by a large, thick-walled, highly muscular and pear shaped structure, the buccal mass.

 Its wall is provided with several sets of muscles for its movement and the movement of radula

a) Buccal musculature: Out of several sets of muscles, the protractors are well developed. They include

i) a median dorsal-three pairs of anterior dorso-laterals and two pairs of posterior dorso-laterals on the dorsal surface and
ii) three anterior muscles and
iii) a pair of long and strong latero-ventral forward muscles on the ventral surface.

 These muscles are mainly concerned with the protrusion and depression of the buccal mass.

b) Vestibule and jaws: Buccal cavity is regionated into an anterior tubular part called vestibule, and a posterior part.

 The posterior limit of the short vestibule is marked by a pair of thickened jaws, placed dorso-laterally one on each side and connected together by a thin cuticular membrane.

 Anterior cutting edge of each jaw is truncated and serrated, bearing numerous small and two or three large tooth-like processes.

 Wall of the vestibule is beset with longitudinal muscle fibres that form the mouth sphincter.

 Sphincter regulates the opening of the mouth and operates the jaws at the time of feeding.

c) Odontophore: In the posterior part of the buccal cavity the floor is raised into a thick muscular structure called tongue mass or odontophore.

 Structure is supported by two sets of cartilages

i) a pair of more or less triangular superior cartilages lying below the epithelium at the top of the odontophore

ii) a pair of S-shaped lateral cartilages, with thick ventral edges and thin dorsal edges, lying on the sides.

 Anteriorly the odontophore forms a small process, the sub-radular organ, roofing a narrow space called the sub-lingual cavity.

d)Radula: Buccal cavity contains a brownish, chitinous, curved, ribbon-like structure, called the radula or lingual ribbon.

 Its anterior end bearing a pair of wing-like flaps, runs longitudinally over the summit of the odontophore.

 Its posterior end is lodged in a band-like, 2mm wide 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-radular membrane.

 Dorsal surface of the radula bears teeth arranged in numerous transverse rows.

 Each row contains seven teeth, one central rachidian, and one lateral and two marginals on its either side, giving the formula 2, 1, 1, 1, 2.

 Radula is moved forward and backward on the odontophore for rasping food particles.

 Movements, called chain-saw movements (Huxley), are brought about by protractor and retractor muscles; the radula can even be protruded from the mouth.

 Regular use causes the radula to wear off at the anterior end, but the loss is made good by regular addition of radular material at the posterior end

2) Oesophagus: This is a narrow and long tube emerging dorsally from the buccal mass.

 Running posteriorly for a short distance, it turns to left and enters the visceral mass to open into the stomach

3) stomach: It lies on the left side of the visceral mass, below the pericardium.

 Its cavity is U-shaped which is regionated into a broad posterior cardiac chamber that receives the oesophagus and a narrow anterior pyloric chamber from which the intestine starts.

 Lining of the stomach is folded; folds of the cardiac stomach are low and run from right to left, while those of the pyloric stomach are somewhat prominent and run transversely.

 A short rounded and blind pouch, the caecum, arises from the lower outer wall of the pyloric chamber.

 At the junction of two chambers of the stomach opens a duct from the digestive gland.

4) Intestine: Pyloric stomach is followed by a long and coiled intestine. It runs backward into the visceral mass where it makes 2.5 or 3 coils, between the gonad in front and the digestive gland behind, before joining the rectum.

5) Rectum: It comprises of a thick-walled tube which extends into the branchial chamber of the mantle cavity between the ctenidium and genital duct.

 Its external opening, the anus, lies about 6 mm away from the edge of the right nuchal lobe.

II) Digestive glands

1. Salivary glands. These are two in number and lie on either side of the posterior part of the buccal mass.

 Each gland looks like a branching white mass.

 A duct from each gland enters the muscles of the buccal mass and then opens into the buccal cavity in the area of the dorsal buccal glands.

 Salivary secretion contains mucin-like substance and a carbohydrase enzyme.

2. Digestive gland. A somewhat triangular plate or cone with a convex outer and more or less flattened inner surface occupies the greater part of the coiled visceral mass.

 This structure is a digestive gland (often referred to as the liver or hepatopancreas), which is also coiled and is brownish to dirty green in colour.

 It has two main lobes, smaller in contact with the stomach and larger extending to the apex of the spiral.

 Two separate ducts arise from two lobes which unite together to form a common duct before opening into the stomach.

 These ducts, open into the digestive gland, branch repeatedly and end blindly in a very large number of small tubes the alveoli.

 Alveoli are lined with a digestive epithelium made up of three types of cells
i) secretory cells- secrete a cellulose digesting enzyme
ii) resorptive cells- digest proteins intracellularly
iii) lime cells- store calcium phosphate.

 Semi-digested food enters into these alveoli, where digestion of cellulose and proteins takes place.

3. Oesophageal pouches: A pair of simple, rounded, cream-coloured oesophageal pouches lies below the salivary glands.

 Each pouch opens by a narrow duct at the junction of the buccal cavity and oesophagus.

 These pouches probably secrete digestive enzymes.

4. Buccal glands: These are a pair of glandular areas in the roof of the buccal cavity, a little in front of its junction with the oesophagus.

 Each glandular area consists of two pads, separated by an oblique longitudinal furrow; each pad bears a row of transverse grooves.

 Exact function of these glands is not known. They are probably of the nature of accessory digestive glands.