Il8 TEMPERATURE AND FISH VERTEBRA JORDAN. 



miisciilat development, most liiglily specialized. It is evident that the 

 presence of numerous vertebral joints is essential to the suppleness of 

 body which is the eel's chief source of power. 



So far as known the numbers of vertebni? in eels range from 115 to 

 160, some of the deep-sea eels* having probably higher numbers, if we 

 cau draw inferences from their slender or whip-like forms; but this char- 

 acter may be elusive. 



THE SHARKS. 



The sharks show likewise a very large number of vertebrae, 130 to 

 150 in the species in which they have been counted. In these tishes no 

 comparative study of the vertebrte has been made. The group is a 

 very ancient one in geological time, and in the comparatively few re- 

 maining memliers of the grouj), the vertebrte, in fact the entire skeleton, 

 is in a very primitive condition. Tlie sharks are free-swimming flsbes, 

 and with them as with the eels, flexibility of body is essential to the 

 life they lead. 



VARIATIONS IN FIN- RAYS. 



In some families the number of rays in the dorsal and anal fins is de- 

 pendent on the number of vertebrte. It is therefore subject to the same 

 fluctuations. t This relation is not strictly proportionate, for often a 

 variable number of rays with their interspinal processes will be inter- 

 posed between a pair of vertebr.T. The myotomes or muscular bands 

 on the sides are usually coincident with the number of vertebra?. As, 

 however, these and other characters are dependent on differences in 

 vertebral segmentation, they bear the same relations to temperature 

 that the vertebrae themselves sustain. 



CONCLUSION. 



From the foregoing examples we may conclude that, other things 

 being equal, the numbers of vertebra? are lowest in the shore-fishes of 

 the tropics, and especially in those of local habits, living about rocks 

 and coral reefs. 



The cause of this is to be found in the fact that in these localities the 

 influences of natural selection are most active. The reduction of verte- 

 bra; may be regarded as a phase in the process of specialization which 

 has brought about the typical spiny-rayed fish. 



These influences are most active in the warm, clear waters of tropi- 

 cal shores, because these regions offer conditions most favorable to 

 fish-life, and to the life of the greatest variety of fishes. No fish is ex- 



1 



* Nemichthys, Xetlastortm. 



t Thus iu the Scorpwnida', Sebastes, the Arctic genus has the dorsal rays xv, 13, 

 the vert«l)r:p r2 4- 19. The tropical geuus Seorpwna has the dorsal rays Xll, 10, the 

 vertebr.TP 10 + 14, while the semi tropical gemis Sebasfichthys has the intermediate 

 numbers of dorsal rays xii, 12, and vertebra' 12 + l^- 



I 



