232 



VERTEBRATE ANIMALS. 



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 

 jjrogression in the fish. Two very distinct tyjies of tail are found 

 amongst the fishes. In one of these, f(jund in most living forms, 

 the tail is composed of two nearly equal lobes \\ hich spring from the 

 end of tlje spiue (fig. lliT, A). This form of tail is said to be "homo- 

 cereal." 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 

 unequally lobed, and is said to be " heterocercal " (fig. 167, B). In 

 these forms tlie 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 sup- 

 ported by a series of spine-like bones, which are called "rays." 



Fig. Ifi7. — A, Swnrrl-fish, slmwinffhrnTincercal tail; B, Sturgeon, sho\ving 

 hetei'ocercal t.ail. 



These rays are sometimes simple undivided rays or spines, when 

 they are called " spinous rays" (l(i'), d) ; but in other casus they are 

 botli divided by tian.sverse joints, and .split up into numerous lon- 

 gitudinal branches tc^Niards their extreiiiities, wlien they are spoken 

 of as " soft rays" (Hj."), </'). The soft rays occur in many fish&s in 

 diff'erent fins, but tlicy are invariably ])resent iu the caudal fin or 

 tail. 



As regards the dii/i-stirr .i}/xti;n in fishes, the niouth is usually 

 fvirni.shed with a com])lic.iteil system of teeth, developed not only 

 upmi the jaws, but upon any or every l)one which enters into the 

 composition of the oral eaviiy. The gullet ojiens into a stomach, 

 usually of large size, and its hinder aperture (the pi/Zontf:) is usually 

 furnished with a valve. Immediately beliind the ])yloric opening 

 of the stomach there is usually a variable number of blind tubes 



