GENERAL CHARACTERISTICS 



2685 



Internal 



Skeleton 



are shown in the skeleton represented on p. 2684, and occur in both the median, and 

 paired fins, of which the names are also given in the same illustration. In the 

 median fins the bases of these rays articulate with the interspinal bones, or, in 

 clasmobranchs, with the radial cartilages. The first 

 rays of the pectoral and dorsal fins may be developed 

 into long spines, having the same structure as teeth. 



In the internal skeleton the backbone is 



divisible only into a trunk and caudal 



moiety. In the fringe-finned ganoid 

 fishes the primitive notochord persists, although it may 

 be partly surrounded by rudimental arches; while in 

 the sharks and higher bony fishes the column is divided 

 into segments, forming vertebrae with doubly-cupped 

 bodies. In sharks and rays the arches and bodies of 

 the vertebras remain separate, but in the other groups 

 they are fused together; in the tail, as shown in our 

 figure of the skeleton of the perch, there is also an 

 inferior arch and spine to each vertebra. In the more 

 primitive fishes the notochord is continued to the 

 hinder extremity of the body, where it is surrounded 

 symmetricall} 7 - by the rays of the caudal fin; this type, 

 which is shown in the accompanying figure of the 

 skeleton of an extinct fringe-finned shark, being 

 termed the fringe-tailed, or diphycercal. Whereas in 

 some fishes with this type of tail the fringes on the 

 upper and lower portions of the caudal fin are of 

 nearly equal depth, in others the lower fringe of rays 

 becomes somewhat deeper than the others, and a 

 further development of this inequality results in the 

 partially-forked or heterocercal tail of the modern 

 sharks and sturgeons, where the end of the backbone 

 is bent upward into the longer superior lobe of the 

 tail, the lower lobe of which is formed exclusively 

 of rays. The lungfishes and sharks have never 

 advanced beyond one or other of these types; but 

 the bony fishes and ganoids, which started with the 

 primitive fringed lobate type, by a gradual shortening 

 of the central part of the tail fin, accompanied by an 

 increasing development of the rays on its lower side, 

 have evolved the completely-forked or homocercal tail 

 of the perch, in which, as shown in the figure, the 

 backbone stops short of the fin rays, and ends in an 

 expanded, unsymmetrical extremity, from which these rays are given off in a fan- 

 like manner, so as to produce an appearance of perfect symmetry in the whole 

 structure. 



