CAUDAL DIPLOSPOKDTLT OF SHAEKS. 55 



disappointing ; for, in the absence of the muscles, the only 

 means of deciding th-e principal metamerism of the tail is by the 

 nerve-foramina, and these could not be made out in any single 

 instance. Yet, judging from the centra of the tail . being 

 markedly shorter, in proportion to their height, than those in 

 the trunk region, it is by no means improbable that the diplo- 

 spondylous condition of the tail is of considerable geological 

 antiquity. 



Embryology and palaeontology both failing us in our efforts 

 to divine the cause and origin of diplospondyly in Sharks, we are 

 constrained to fall back upon the evidence afforded by the tran- 

 sitional vertebrae, and upon another important fact, that diplo- 

 spondyly does not extend to the extreme posterior end of the 

 vertebral column. The only reference to this fact that I have 

 been able to discover in the scattered literature of the subject is 

 the remark by Mayer (13. p. 267), " Somit entspricht an der 

 Schwanzspitze wenigstens jedem Myotom ein Sclerotom." In 

 the hinder three-fourths of the caudal fin of Acanthias the 

 myomeres are as numerous as the centra. The change from the 

 diplospondylous to the monospondylous condition occurs at 

 about the twenty-fourth centrum from the end ; but the rela- 

 tions between the vertebrae and the muscle-segments can only be 

 made out for the anterior half of these ; for in the hinder part 

 there is scarcely any muscle at all between the skin and the 

 vertebrae. The last ten or twelve vertebrae are imperforate, as 

 already shown to be the case in Scyllmin by von Iheriug and 

 Mayer (9. p. 228, and 13. p. 269), and the little muscular tissue 

 that is attached to these vertebrae is innervated by a backward 

 extension of the nerves supplying the preceding myomeres. 



Diplospondyly is thus confined to that part of the body lying 

 between the cloaca and the greater part of the caudal fin ; and 

 the questions that most naturally present themselves are these — 

 What advantage does diplospondyly confer on this part of the 

 body, and in what respect would this part suffer if the mono- 

 spondylous condition prevailed ? The answer, it seems to me, 

 can be given in a single word — flexibility. Those who have 

 watched dogfish swimming in an aquarium will know how im- 

 portant is the "tail,"' or post-cloacal region of the body, as an 

 organ of locomotion, the paired fins playing but a small part in 

 the actual progression of the body through the water. Tet, 

 when the tail is lashed from side to side, the caudal fin at its 



