Anatomy of finfish and shellfish

Unit 13- Mollusca

13.4.3. Digestive System
Alimentary canal
The mouth, lying in the midst of the oral arms, is surrounded by a fleshy, circular lip, beset with numerous papillae. Just within the lip is a pair of sharp, powerful, horny jaws, looking like the inverted beak of a parrot. The mouth leads into a large thick-walled, muscular pharynx or buccal cavity, containing tongue or odontophore and the raaula. The oesophagus is a long narrow tube running straight backwards between the two lobes of the liver to open into a rounded thick-walled muscular bag, the stomach. A large thin walled and slightly- coiled pouch, the caecum, is con¬nected to the stomach close to the starting point of the intestine. The short intestine runs anteriorly nearly parallel with the oesophagus and merges into the rectum which opens into the mantle cavity by the anus. A pair' of leaf-like anal valves, of uncertain function, project at the sides of the anus.

Fig. 5. Sepia. Alimentary canal

Digestive glands
Sublingual glandular tissue of unknown function lies on the ventral side of the tongue. A pair of anterior salivary glands lies within the buccal mass opening on either side of the radula. Pair’s of posterior salivary glands, lying in front of the liver, one on either side of the oesophagus, open by a common duct at the tip of the tongue in buccal cavity. The salivary glands have been misnamed because they are an really poison glands and their secretion is used to paralyse the prey. The large, brown digestive gland or liver extends from near the posterior end of the body. It is a solid, btobed gland, giving off one duct from each lobe. The two ducts pass through Ink-duct' the dorsal renal chamber and unite to open into the vesti¬bule or the chamber where the stomach, the caecum sac and the intestine meet. The two hepatopancreatic ducts bear minute vesicles which constitute the pancreas.
Ink gland
As already stat¬ed, a pear-shaped ink sac lies over the posterior ventral sur¬face of visceral dome and opens by a duct dorsally into the rectum close to the anus. The terminal part of the duct forms on ejecting ampulla. The ink gland, lying inside the wall of the large reservoir or ink sac, secretes a brown or black fluid or ink. It contains a high concentration of melanin pigment and is stored in the ink sac. When the cuttlefish is startled, it discharges the ink through the funnel as a black cloud, which forms a sort of smoke-scren or a dummy, under the cover of which the animal escapes from an enemy or approaches a prey. The ink of Sepia provides sepia pigment used by artists for hundreds of years.

Fig. 6. Sepia. Ink-sec in M.L.S.

Food and feeding mechanism
All cephalopods are carni¬vorus. The food consists chiefly of Crustacea, Mollusca and small fish. The tentacles are rapidly extended and attached to the living prey by the suckers. Then the tentacles retract, so that the food is brought within the reach of the arms which hold it. The prey is paralysed by the poisonous secretion of the salivary glands, broken into pieces by the jaws and swallowed probably by the aid of the radula.
Digestion
Inside the stomach, the food mixes with the fluids from the liver and pancrease. The semi-digested food passes to the spiral caecum, where digetion is completed. The liquid products of digestion are absorbed in the caccum, while undigested food is passed on to the intestine where some absorption may take place.- The residual food is expelled out of the anus. -
The hepatoancreas not only secretes digestive enzymes but also serves for the absorption and storage of food and the excretion of waste products. The cephalopods differ from the majority of invertebrates in that probably the food stored in the liver is not absorbed directly but received through the blood by the liver.