9 Of 

 OA 



PISCES. 



tube passing directly from the stomach to the 

 anus. In the Mullets ( Mitgil), on the con- 

 trary, the intestine is of great length and is very 

 curiously folded, being intervolved with the 

 folds of the liver so as to form a mass of trian- 

 gular form moulded to the shape of the abdo- 

 minal cavity, and affording an example of the 

 longest and, in its disposition, the most complex 

 intestinal canal of any of the class. 



This arrangement, as Kathke very justly ob- 

 serves, is an obvious approximation to the dis- 

 position of the viscera most generally met with 

 in molluscous animals, in which the folds of 

 the intestine are prolonged into and almost bu- 

 ried among the folds of their very large liver. 



In the Electric Eel (Gymnotus elcctricus) 

 the disposition and termination of the intestinal 

 tube is curious. First descending to the lower 

 end of the stomach, it passes to the left side 

 and ascends again as far as the oesophagus ; it 

 then winds downwards and backwards so as to 

 encircle the stomach, and, lastly, advancing for- 

 wards along the ventral aspect of the abdomen, 

 it terminates, as in the Cephalopoda, beneath 

 the throat, in the immediate vicinity of the 

 heart and root of the tongue. 



No fish has anything like a colon or ccecum. 

 The only distinction between small and large 

 intestines is met with just at the termination of 

 the alimentary tube, where it opens into a kind 

 of cloacal cavity, usually called the rectum. At 

 this point there is generally a prominent cir- 

 cular fold of the lining membrane constituting 

 a kind of valve. In the Salmon several of these 

 valvular zones succeed each other, giving to this 

 part of the gut an appearance similar to that of 

 the intestine of the Plagiostome cartilaginous 

 Pishes immediately to be described. 



In the Sharks and Rays, the Plagiostomes 



Fig. 519. 



Alimentary canal of Sftark. 



a, oesophagus ; b, b, cavity of stomach, at the 

 commencement of which are placed the valvular 

 fringes mentioned in the text ; c, passage leading to 

 pylorus ; d, spleen ; e, pyloric cavity ; f, dilated 

 chamber; g, h, bile-ducts; t, orifice of pancreatic 

 duct -, k f k, valvular intestine. 



just referred to, and also in the Sturgeon, the 

 intestine presents a very remarkable structure. 

 Externally it resembles a wide bag nearly simi- 

 lar in shape and size to the stomach itself, and 

 so short and stunted that, without some special 

 arrangement, obviously a sufficient surface 

 would not be afforded for the absorption of the 

 nutritious portions of the food. By the me- 

 chanism adopted, however, this is abundantly 

 provided. Throughout the whole length of the 

 gut the mucous membrane is arranged in deep 

 spiral folds (Jig. 519), which wind from end to 

 end, only leaving a small orifice in the centre of 

 each valvular projection, whereby the different 

 compartments formed between the spiral lamina 

 can communicate with each other, so that the 

 digested food by this unusual arrangement is 

 spread over a very great superficial area, and all 

 the benefits of a long and convoluted intestinal 

 tube are secured. Each fold of this extensive 

 spiral valve contains between the layers of mu- 

 cous membrane that compose it an elastic 

 substance, whereby it is kept constantly spread 

 out and restored to its original position when 

 displaced by the passage of food through the 

 central channel that permeates the whole series. 



In the Sturgeons a similar valve exists, but 

 its spiral folds are not so closely arranged : the 

 intestine, moreover, is remarkable on account 

 of the great thickness of its muscular and in- 

 ternal tunics, the latter of which presents a 

 reticulated or honey-combed appearance, the 

 larger meshes including irregular spaces, which 

 are again subdivided into smaller cells. Slight 

 vestiges of the spiral intestinal valve are visible 

 even in the Lamprey. 



Salivary glands. From the circumstances 

 under which Fishes swallow their food, the 

 presence of any salivary apparatus is evidently 

 uncalled for; no fish, therefore, possesses true 

 salivary glands. Nevertheless, in the Cypri- 

 nidse and some other races the whole palate is 

 covered with a soft spongy substance, from 

 which a kind of mucosity is discharged through 

 imperceptible pores, which has been regarded 

 by Rathke and others as a salivary organ : 

 Cuvier, however, denies the glandular character 

 of this substance, regarding it as a peculiar and 

 highly sensible tissue destined to be the seat of 

 a sense more or less analogous to that of taste, 

 a supposition that is rendered more probable 

 from the great number of nerves that enter its 

 substance. 



Pancreas. In the osseous Fishes no pan- 

 creas, such as that met with in the higher 

 classes of vertebrate animals, exists ; it is, 

 I however, represented by a variable number of 

 ccecal appendages, which open into the duode- 

 num in the vicinity of the pylorus. The lining 

 membrane of these pyloric cceca is of a glan- 

 dular character, and secretes an abundance of 

 a thin glairy fluid analogous to the pancreatic 

 secretion. The existence of the appendages in 

 question is, however, by no means constant; 

 thus in the Labridce, the Siluridce, the Cypri- 

 nidee, and many members of the pike genus, 

 they are altogether wanting. When present, 

 moreover, their number varies very remarkably 

 in different Fishes ; thus, sometimes, as in the 



