INTERNAL ORGANS 



171 



one Shark the spiral valve has as many as forty turns, whilst 

 in some of the Hammer-heads (Sphyrna) it has the appearance 

 of a scroll. The pecuhar and characteristic shape of the 

 fossilised faeces ("coprolites"), known to have been excreted 

 by extinct shark-like fishes, indicate that 

 these forms also possessed a spiral valve. 

 The function of this structure is to in- 

 crease the area of absorptive surface, 

 an end which is accomplished in the 

 Bony Fishes by an increase in the length 

 of the intestine. A more or less vestigial 

 valve is found in the Sturgeons (Acipenser), 

 Bichirs (Polypterus), Lung-fishes (Dip- 

 neusti), Bow-fins {Amia), and other primi- 

 tive forms, but this disappears in all the 

 higher Bony Fishes. 



In the Selachians and Lung-fishes the 

 rectum opens into a cloaca, which also 

 receives the ducts from the kidneys and 

 reproductive organs, but in all the re- 

 maining Bony Fishes it opens to the exterior 

 by the vent or anus, lying in front of the 

 excretory and reproductive openings. The 

 cloaca is invariably situated near the 

 junction between the trunk and tail 

 regions of the body: the vent, on the 

 other hand, varies considerably in position 

 in different fishes, and may occupy almost 

 any position from the primitive one at the 

 hinder end of the trunk to one between 

 or even in front of the pectoral fins. In 

 the Electric Eel {Electrophorus) and other 

 Gymnotid fishes the vent is actually to be 

 found in the throat. 



Space will not permit of a description 

 of the elaborate and delicate structure of opened to show spiral 

 the lining membrane of the different parts \a \e, x 



of the alimentary canal, but it may be pointed out that this 

 is, for the most part, designed either to prepare the food for 

 absorption into the blood or to carry out the absorpti\'e 

 process itself. Like any other animal, the fish, in order to live, 

 has to convert the food into power, and after the food is 

 dissolved in the stomach and intestines the nutritious part is 



Fig. 69. 



Rectum or large intes- 

 tine of Ray {Raia sp.), 



