170 A HISTORY OF FISHES 



is reduced to a mere crack, and is lined by a thick horny 

 covering. 



Passing from the stomach, the food, which is now in a Hquid 

 condition, enters the intestine, the commencement of which 

 is generally marked by the presence of a ring-like thickening 

 (pyloric valve) of the inner surface of the canal, or, if this is 

 wanting, by the entrance of the ducts leading from the liver 

 and pancreas (Fig. 68). The purpose of this section of the 

 alimentary tract is connected with the absorption of the food 

 into the blood, the essential process of assimilation, and the 

 length of the intestine in a particular fish is closely connected 

 with the nature of its normal diet. In the Sharks and Rays, 

 and in many of the Bony Fishes feeding largely on other fishes, 

 the intestine is straight, or at the most is thrown into one or two 

 simple loops, but in the vegetarians and mud-eaters it is 

 exceedingly long and variously coiled and looped, so as to 

 pack the maximum of absorptive surface into the minimum of 

 space. In the Grey Mullets (Mugilidae), for example, it is very 

 lengthy and closely coiled; in the Stone Roller {Campostoma) , a 

 member of the family of North American Suckers {Catostomidae) , 

 it is wound round and round the air-bladder, and in the 

 Mailed Cat-fishes {Loricariidae) of South America it is disposed 

 in numerous spiral coils like the spring of a watch. Mention 

 may be made here of two important glands pouring their 

 juices into that part of the intestine which lies immediately 

 behind the stomach, and which play their part in the process 

 of digestion. These are the liver, a large irregular mass of tissue 

 varying much in size and colour in different fishes, and generally 

 provided with a gall-bladder as in higher vertebrates, and the 

 pancreas, a more diffuse gland, a part of which is generally 

 more or less embedded in the substance of the liver (Fig. 68). 

 Sometimes the products of liver and pancreas are carried to the 

 intestine by a common duct. Another dark red gland, the 

 spleen, is found attached to the stomach in practically all 

 fishes. 



The rectum or large intestine, the last part of the alimentary 

 canal, may be recognised by its straight course to the vent or 

 sometimes by an increase in calibre. In the Sharks and Rays 

 this develops a curious internal structure known as the spiral 

 valve (Fig. 69). This occurs in its simplest form in the 

 Lampreys [Cyclostomes), but attains its maximum development 

 in the Selachians, where it may be very complicated and 

 exhibits a good deal of variation in the different species. In 



