Anatomy of finfish and shellfish

Unit 13 - Mollusca

13.2.11. Digestive System
The digestive system consists of an alimentary canal and a paired digestive gland.
Alimentary canal
It is a long and coiled tube of varying diameter, consisting of the following parts.
Month and labial palps
The mouth is a transverse slit, lying in the middle line between the anterior adductor muscle and the foot. It is bounded on each side by two, somewhat oval, fleshy flaps, the inner and outer labial palps. The cilia on the surface of these palps drive the food particles into the mouth. The two labial palps on each side enclose a ciliated oral groove leading into mouth. The mouth lies between two ridges or lips, the anterior lip connecting the two outer palps, and the posterior lip connecting the two inner palps of the opposite sides. The mouth has no teeth.

Fig. 14. Unio. Digestive system dissected from left side

Oesophagus
The mouth leads directly into a short tube, the oesophagus. A radula is absent, being useless to the animal that feeds only on minute organisms. The inner wall of oesophagus is ciliated.
Stomach and style sac
The oesophagus opens dorsally into a wide, thick-wailed oval sac, the stomach, surrounded by a large paired digestive gland or liver and connected with it by several ducts. The posterior end of stomach gives off ventrally a tubular diverticulum, the pyloric caecurn or the style sac. It contains a transparent, solid, flexible and gelatinous rod, the crystalline style. It is secreted in concentric layers by the ciliated epithelium lining the style sac and contains a mucop¬rotein and a cellulose and starch digesting enzyme. The cilia if the style-pouch rotate and move the style forward, so that its free end, projecting into the stomach, is constantly rubbed against a special portion of stomach wall, called the gastric shield. Thus, the head of the style is constantly worn away and its substance is mixed with the contents of the stomach.
Intestine
The intestine arises from the stomach floor in front of the style sac. It runs ventrally into the foot, where it is coiled upon itself through the visceral mass, much of which is the yellow-coloured branched gonad. It then runs dorsally again and is continued into the rectum.
Rectum
The rectum, or terminal part of the intestine, runs posteriorly through the pericardium, traversing the ventricle which is actually wrapped around it. Finally, it ends in an anal papilla which opens in the cloacal chamber above the posterior adductor muscle by the anus. Internally, the rectum has afolded mid-ventral ridge, like the typhiosole of the earthworm.
Digestive gland
The paired digestive gland or liver of the freshwater mussel is a large, dark brown or green gland of an irregular shape, surrounding the stomach. It is made of highly branched tubules, lined by columnar epithelial cells of unequal height and containing basal nuclei. It opens into anterior end of the stomach by many ducts, which are lined with ciliated epithelium. The gland not only produces digestive ferments, but its cells readily ingest and break down solid foot particles, digest proteins and fats intraceliularly, and absorb carbohydrates. The gland is also regarded to be excretory by some people.
Food and feeding mechanism
The food consists of diatoms Protozoa other planktonic micro-organisms and organic detritus brought in by the respiratory water currents. The mussel is typical filter feeder and the ctenidia also serve the function to obtain food. It is a sluggish creature, incapable of actively seizing its prey. Therefore, it obtains its food through current set in water by ciliary movements, in the manner of sedentary organisms. Much like the sessile sponge, mussel must pump large quantities of water through its body, in order to extract food and oxygen and to get rid of the wastes.

Fig. 15. Unio. Feeding Mechanism

The beating of lateral cilia borne on the outer surface of gill fluent and coca precut on tike inner surface of tile mantle draws a constant water current, through the indolent siphon into the mantle cavity with microscopic plants lad animals suspended in it.
The particles are strained out as the water passes through the gills. The heavier sand particles are simply dropped from the surface of the galls to the edge of the mantle, driven backwards by the cilia on tile mantle, and expelled out posteriorly. The lighter food particles are thrown by the latero-frontal citron to the outer lamellar surfaces, where they become entangled in mucus secreted by the galls. The food-laden mucosa mass from both sides of each gill Lima are moved ventrally by beating of the frontal calla into the food groove at the lower edge of the gill lamina. Cilia of the food groove drive tie food I parolees forward towards tie mouth When the labial palps are reached further sorting takes place, m an unknown way, according to the nature of the particles. The large and indigestible particles, under the influence of the cilia working along rejection paths, drop down and get removed from the mantle cavity with the outgoing circulation on the mantle at the posterior end throngs the incurring siphon. The call and digestible particles are (tarried straight into the mouth, along the deep collate groove between time two labial palps, leading to a corner of the mouth.
Digestions
Tie physiology of digestion in freshwater mussels is not known. Its knowledge is mainly basal on be smiles of some marine clams. The food is mostly digested and parry absorbed in the stomach. As the feed is microscopic, tie digestion is mostly intercellular by phagocytes lining tie alimentary canal and by wandering leukocytes which play a truth part in the teleport of food all over the body.
Food from stomach esters the digestive gland, the cells of which have a surprising power of ingests solid particles, lid absorbed digesting protists and fats intracellularly, carbohydrates. Folds of the stomach wall around the apertures of the ducts of digestive gland permit only tie finest food particles to enter the gland. According to Owen (1955) the duds of the digestive gland show two kinds of track, non- ciliated for receiving tie food particles and ciliated for sending time undigested wastes back into the stomach. Extra-cellular digestion also occurs in tile stomach. A digestive fluid is secreted by to digestive gland and liberated m the stomach by ducts. It is said to contain the enzyme amylase. The crystalline cone also helps in extra-cellular digestion in two ways.