the corpus, but the latter can be easily distinguished

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- 79 - ALH1ENTARY CANAL Banki (1936) broadly divides the alimentary canal of fishes into two regions, the 'Kopfdarm' comprising the buccal ca.

1 ALH1ENTARY CANAL Banki (1936) broadly divides the alimentary canal of fishes into two regions, the 'Kopfdarm' comprising the buccal ca.vity and the pharynx and the 'rumpfdarm' comprising the foregut (oesophagus and stom~~h), midgut (intestine) and hind gut (rectum). The relative leng~hs of the different regions vary considerably depending on the food habits of the different groups. It is longer in herbivorous than in carnivorous fishes.. The gross anatomy of the alimentary canal in fishes was worked out by -Munro (1795), Home and Everade (1814), Rathke (1824), Banki (1936) and S~yehiro (1942) and its histology in representative types have been worked out in detail by Muller,J (1843), Agassiz and Vogt. (1845), Sullivan (1907), Greene (1912), Ben Dawes (1929), Irving H.B1ake (1930), Mary Dora Roglck (1931) and Curry (1939). In a typical teleost such a Mugi1 (Ref.Suyehiro, 1942) the stoma.ch is spacious, the intestine is long and coiled and the vent is located in the middle of the ventral side. But in the eels and eel-like fishes, the elongation of the body has re~ulted in a corresponding elongation and straightening o~ the intestine. The mouth is terminal and has a wide gape in A,bicolor, but in P.boro, S.bengalensis and A,fossorius the gape is comparatively narrow. In A.bicolor the buccal cavity is spacious, while in the other three species, it is comparatively narrow.

- 80 - The lips are thick, except in A.fossorius. The upper lip is supported by t~e maxillae which are comparatively thin in all the four species. The premaxillae are present only in S.

2 The lips are thick, except in A.fossorius. The upper lip is supported by t~e maxillae which are comparatively thin in all the four species. The premaxillae are present only in S.bengalensls and A.fossorius. Maxi- llary~nd species. mandibular teeth are present in all the fo~r Palatine. teeth are present in S.bengalensis and A.fossorius. and vomerine teeth in A.bicolor and P.boro. The tongue is free and prominent in A.bicolor, but in the other three species, it is in the form of a median elevation of the floor of the buccal cavity (Figs. 147, 149, 151 & 15'~). The palate i s flat and firm, but the floor of the buccal cavity can be raised or lowered like bellows to draw in air for respiration. The pharynx is short and carries five gill-sllt~, the first pair being invariably the largest. Both the upper and lower pharyngeals are present;the lower pharyngeals are elongated bony plates and the upper pharyngeals are rounded plates carrying small pointed teeth. The oesophagus is short and muscular with 7-12 longitudinal ridges leading into the stomach. In A.bic~ior the hind end of the oesophagus is narrow. The opening of the pneumatic duct into the oesophagus'. is seen'as a minute pore on the mid-dorsal line in P.boro, but is occluded in A.bicolor. The air-bladder is absent in S.bengalensis and ~.fos8oriu8. The teleostean stomach is usually divided into two regions, the 'cardiac' and the 'pyloric', but accord-

- 81 - ing to Barrington (1957) ~hese terms are misleading and therefore suggests that the two regions of the stomach may be called the 'corpus' and 'pyloric' regions.

3 ing to Barrington (1957) ~hese terms are misleading and therefore suggests that the two regions of the stomach may be called the 'corpus' and 'pyloric' regions. The caecum corresponds to the corpus and the pyloric region is ~ short tube which connects the corpus with the intestine,, but in S.bengalensis a~d A.fossorius the differentiation into corpus and pyloric regio~s is absent. On the other hand, in'a.bicolor and F.boro both regions are distinguishable. In A.bicolor (Fig. 148), the longitudinal ridges of the oesophagus are continuous with the ridges in / the corpus, but the latter can be easily distinguished from the former. The ridges in the oesophagus are straight, but in the corpus, they are thick and wavy in the anterior half while posteriorly they are narrow, straight and converging towards the apex. Some of the ridges on the right side of the corpus extend into the pylorus. In F.boro (Fig. 150), the straight longitudinal ridges of the oesophagus are continued uninterrupted into the anterior third of the corpus, the middle part is smooth and devoid of ridges, but they are again visible in the posterior third. Unlike the condition in A.bicolor, these ridges do not extend into the pylorus. In both these species, the pylorus is situated on the right side of the junction between the stomach and the intestine. In S.bengalensis and A.fossorius, the pylorus is absent (Figs. 152,& 154), the stomach being continuous with the intestine, though demarcated from the latter by a pyloric sphincter.

- 82 - In all the four species, the intestine is comparatively short and is less than half the total length of the digestive tube. In A.bicolor and P.boro (Figs.

4 In all the four species, the intestine is comparatively short and is less than half the total length of the digestive tube. In A.bicolor and P.boro (Figs. 148 & 150), the lining of the intestine is thrown into numerous delicate folds, which extends along the entire length, but in S.bengalensis and A.fossorius (Figs.l5~ & 154) such folds are prominent only in its anterior half. In all the four species, the rectum is short and is not clearly demarcated from the intestine externally, but. internally an intestino-rectal valve is present.in all the four species longitudinal ridges are present on the inner lining of the rectum. In A.bicolor and P.boro the liver is situated on the ventral side of the oesophagus. asymmetrioally ~ilobed It is an elongated, structure, extending from the middle of tlle oesophagus to the anterior part of the intestine. In A.bicolor (Fig.147), the left lobe extends backwards to the anterior part of the stomach, but in P.22rQ (Fig. 149) it reaches only up to the junction between the oesophagus and the stomach. In S.bengalensis and A.fossorius (Figs. 152 & 154), there is only a single elongated liver mass extending from the anterior end of the stomach to the hind end of the intestine and closely adhering to its right side. From its position in relation to the intestine, this may be regarded as the left lobe of the liver. However, in A.fosBorius, from the middle of the liver, immediately behind the pyloric sphincter arises a small narrow extension of the liver mass which

- 83 - projects at right angles to its long axis. This lobe-like extension runs beneath the. pylorus and curves upwards on the right side of the intestine (Fig. 153).

5 projects at right angles to its long axis. This lobe-like extension runs beneath the. pylorus and curves upwards on the right side of the intestine (Fig. 153). It is possible that this may represent the reduced right lobe, in w~ich case it has to be presumed that the reduced condition of the right lobe is the result of the gi"eate:r: development and elongation of the left lobe. The liver is thickest in the middle and becomes gradually narrow towards the two ends. The gall bladder is located in front of the pyloric region and in between the liver and the intestine. In all the four species, the pancreas is of the diffuse type and the pancreatic tissue lies scattered in the liver mass and partly surrounds the hapatic portal vein. The histological structure of the different layers of the alimentary canal in the four species described in this paper are similar in all essential details to that in other teleosts such as Pleuronectes platessa (Ben Dewes, 1930) and so only the noteworthy differences met with in these species of fishes are described. 'In all the four species the mucosa of the buccal cavity (Figs. 155, 156, 157 & 158) is a very thin layer of cuboid cells with scattered mucous cells. The folds of the. mucosa are low in A.bicolor and P.boro, whereas in S.benga1ensis and A.fossorius they are conspicuous. all the four species, the cytoplasm o~ is clear and the nuclei are small and compact. In the cuboid cells ~11 S.bengalensis and A.fossorius there isaabundance of blood capillaries in this region and most of them reach the inner surface of the mucosa. In

- 84 - In A.fossorius taste buds (Fig. 158) are. present in the mucosa of the buccal cavity. They are similar in structure to those in Pleuronectes platessa.

6 In A.fossorius taste buds (Fig. 158) are. present in the mucosa of the buccal cavity. They are similar in structure to those in Pleuronectes platessa. Each taste-bud consists of six to eight elongated I spindle-shaped cells with centrally placed nuclei. Each taste-bud opens into the buccal cavity through a minute pore, called the gustatory pore. The mucosa of the pharynx (Figs. 159, 160, 161 and 162) consists of three to four layers of small cuboid cells in A.bicolor and S.bengalensis whereas in Foboro and A.fossorius, it con~ists of only a single layer of large cuboid cells with vocuolated cytoplasm and centrally placed nuclei. In all the four species, these cuboid. cells are interspersed with goblet cells. In A.fossorius this layer is highly vascular. The mucosa of the oesophagus (Figs. 163, 164, 165 & 166) is in all the four species thrown into longitudinal folds which are much more prominent than in the pharyngeal region and are continued posteriorly without interruption into the stomach. The mucosa consists of a single layer of large quboid cells, amo~~ which are seen large scattered mucous(cells of the goblet type. In Anguilla oicolor, F.boro and S.bengalensis the mucosa of the stomach is folded into simple tubular glands (Figs. 167, 168 & 169)0 In A.fossorius (Fig.170) the longitudinal folds are absent in the mucosa of the stomach their place being taken up by numerous punch-like inpushings. The mucosa of the intestine of all the four species is thrown into longitudinal ridges. In A.bicolor

1 I -85 - and P.boro, these folds carry numerous secondary folds. In all the four species (Figs. 171, 172, 173, 174 and 175) the mucosa consists of a single row of columnar cells.

7 1 I and P.boro, these folds carry numerous secondary folds. In all the four species (Figs. 171, 172, 173, 174 and 175) the mucosa consists of a single row of columnar cells. The cytoplasm of these cells takes a darker stain with eosin and acid fuchsin near the inner free margin. This deeplv stainin~ marginal part is d9s~rihed as thp. topplate. The rest of- the cytoplasm takes a uniform but lighter stain. The nuclei are placed near the base. Scattered among these cells, there are numerous goblet cells, except in A.fossorius. In A.bicolor and P.boro, small interstitial cells are present at the base of the columnar cells. The mucosa of the rectum (Figs. 176, 177, 178 & 179) consists of a single row of cells as in 'the intestine, but the ridges are much lower than in the intestine. In P.boro and Sobengalansis, top-plates similar to those found in the mucosa of the intestine, are present in the rectum also. The submucous layer has a remarkably uniform structure through out the different regions in all the four species. This layer consists of a loose mass of areolar connective tissue with numerous blood vessels. Where ridges are present in vhe mucosa, the submucous 1aye~ extends into each ridge. Though the muscle layers exhibit uniformity in structure in all the four species, its thickness varies considerably in the different regions and in different species.

- 86 - The muscle layers are absent in the buccal region. The pharynx in A.bicolor, P.boro and S.

8 The muscle layers are absent in the buccal region. The pharynx in A.bicolor, P.boro and S.bengaiensis is enveloped by a comparatively thick layer of circular muscles and a few strands of longitudinal muscle fibres. In A.bicolor and A.fossorius both the circular and longitudinal layers of muscles are present but in P.boro the longitudinal muscles and in S.bengalensis, the circular muscles are absent. The circular muscles form a thick and compact layer in the caecum of the stomach in A.bicolor and Poboro. In both these species, the outer layer of longitudinal I muscles is very thin. In S.bengalensis and A.fossorius, the wall of the stomach contains only a thin, but compact layer of circular muscles. and A.fossorius, In the intestinal wall of P.boro, S.bengalensis there is only a single compact layer of circular muscles, but in A.bicolor, both the circular and longitudinal layers of muscles are present. In the rectum of all the four species, circular muscles from a compact, the' comparatively thick layer which gradually increase in thickness towards the hind end. Longitudinal muscles are present only in A.bicolor. The scrosa has the same structure as in Pleuronectes platessa. The liver is enclosed in a thin layer of con~ nective tissue, the mesothelium, which 'is continuous',"with the peritoneum. The hepatic cells (Figs. 180, 181, 182 '.~ and 183) are regular and polyhedral with vacuolated cyto-

- 87 - plasm and fairly large centrally placed nuclei. These cells are arranged in a more or less regular order around the hepatic ductules.

9 plasm and fairly large centrally placed nuclei. These cells are arranged in a more or less regular order around the hepatic ductules. The bile ductules are seen as thinwalled canals amqng the hepatic cells and these ductules join together to fdrm larger ducts which have slightly thicker walls. The capillaries in the liver mass which are often termed the hepatic simusoids are irregular, thinwalled channels through which blood can move slowly, thereby permit~ing diffusion to take place between the plasma.' and the hepatic cells, immediately outside the sinusoidfi. The simusoids anastomose and converge towards the centre of the liver mass, where they emply into larger veins. Mecrophages are often seen in the sinusoids. They are very small cells with oval nuclei and are surrounded by a very thin film of cytoplasm vjhich is hardly d~stinguishable. These cells are also termed Kupffer's cells. The pancreas (Figs.184, 185, 186 & 187) surrounds the hepatic portal vein close to the anterior end of the intestine. It is enveloped by a thin layer of connective tissue, the strands of which divide the pancreas into several acini. The greater part of the gland consists of pancreatic acinar cells and are of the nature of exocrine gland cells. These are large cells with c.lear cytoplasm containing numerous inclusions which take a deep red colour with Mallory's triplestain. not clearly visible, eacept in I.boro. The nuclei are Pancreatic ductules and blood capillaries are scattered among these

- 88 - cella. There are also a few aggregations of cells which constitute the islets of Langerhahso Each islet is enclosed in a thin sheath of connective tissue.

10 cella. There are also a few aggregations of cells which constitute the islets of Langerhahso Each islet is enclosed in a thin sheath of connective tissue. The individual cells are more or less oval and the nuclei take a dark stain with Ehrlich's haematoxylin. The cytoplasm does not stain properly with Mallory's triple stain. There is a rich network of blood capillaries among these cells. The islet cells constitute the endocrine portion of the gland. Such islet cells have not been observed in P.boro.

UROMASTIX DIGESTIVE SYSTEM

UROMASTIX DIGESTIVE SYSTEM by Dr. Rashmi Tripathi Department of Zoology Brahmanand College, Kanpur DIGESTIVE SYSTEM : The digestive system consists of (A) Alimentary canal and (B) Associated digestive