210 



VERTEBRATE ANIMALS. 



a Homocercal tail (Sword-fish) ; b 

 Heterocercal tail (Sturgeon). 



Variable in number, and in some cases there is only a single 



fringe running round the hinder 

 extremity of the body. Common- 

 ly, however, the median fins con- 

 sist of one or two expansions of 

 the dorsal integument, called the 

 "dorsal" fins (Fig. 101, d d') ; one 

 or two on the ventral or lower 

 surface near the vent, called the 

 " anal " fins (a) ; and a broad fin 

 at the extremity of the vertebral 

 column, constituting ihe " caudal " 

 fin or tail (c). 



The tail in all fishes is placed 

 vertically that is to say, it strikes 

 the water laterally, or from side to 

 side, and it is the chief organ of 



FIG. 103. Tails of different Fishes, progression in the fish. Two very 



distinct types of tail are found 

 among the fishes. In one of these, 

 found in most living forms, the tail is composed of two nearly 

 equal lobes which spring from the end of the spine (Fig. 103, 

 a). This form of tail is said to be "homocercal." In the 

 other type of tail, found in the dog-fishes, sharks, and other 

 living fishes, as well as in many extinct forms, the tail is un- 

 equally lobed, and is said to be " heterocercal " (Fig. 103, I). 

 In these forms the vertebral column is prolonged into the 

 upper lobe of the tail, and the greater portion of the tail is 

 found below the spine. 



In both the paired and the median fins the integument is 

 supported by a series of spine-like bones, which are called 

 " rays." These rays are sometimes simple undivided rays or 

 spines, when they are called " spinous rays " (Fig. 101, d) ; but 

 in other cases they are both divided by transverse joints, and 

 split up into numerous longitudinal branches toward their ex- 

 tremities, when they are spoken of as " soft rays " (Fig. 101, 

 d'). The soft rays occur in many fishes in different fins, but 

 they are invariably present in the caudal fin or tail. 



As regards the digestive system in fishes, the mouth is 

 Usually furnished with a complicated system of teeth, de- 

 veloped not only upon the jaws, but upon any or every bone 

 which enters into the composition of the oral cavity. The 

 gullet opens into a stomach, usually of large size, and its hin- 

 der aperture (the pylorus) is usually furnished with a valve. 



