232 



VERTEBRATE AKIMALS. 



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 

 progression in the tisli. Two very distinct types (jf tail are found 

 amongst the fishes. In one of these, found in most living forms, 

 the tail is composed of two nearly e(inal lolics which spring from the 

 end of the spine (fig. 167, A). This form of tail is said to be "homo- 

 cercal." In the other tyjie of tail, found in the Dog-fishes, Sharks, 

 and other living fishes, as well as in many extinct forms, the tail is 

 iuie(pially lolie(i, and is said to be " lieterocercal " (fig. 167; B). In 

 these forms the verteTiral 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 



j)aired a 

 ported by a series of 



(1 the median fins the integument is sup- 

 iiine-like bone.s, which are called "rays." 



FJL'. li'7. — A, S\'.'nril-fish, shnwinf^ hnlno^erc.^i tail; R. Stnrtrpnn, slinwinc; 

 lu'tc-roc.-iral tail. 



These rays are sometimes simple undivided rays or spine.s, when 

 they are called " spino\is rays" (10.5, d) ; but in other cases they are 

 both divided by transverse joints, and split up into numerous lon- 

 gitudinal branches tow.uils their extremities, wdien they are spoken 

 of as " soft rays " ( Hio, </'). The .soft r.'iys occur in manj' fi.shes in 

 diffei'ent fins, but they ai'e invari.-[l)ly |)resent in the caudal fin or 

 tail. 



As regai'ds the ilii/r.-il.ir<' ^i/^fnn in iishcs, the month is usually 

 furnished with .i coniplic.itid system of teeth, developed not only 

 npon the jaws, but ujmiu any or eveiy bone whiih enters into the 

 composition of the or;d cavity. The gidlct o|iens into a stomach, 

 usually of large size, and its hinder .-iperture (the pi/hinix) is nsu.ally 

 fuiiiislii'd with ,-1 valve. rmmcdiately behind the pyloric o]>ening 

 of the sIiiiicmIi Ihcre is usu.illv a variable number of blind tubes 



