[6.VJ THE EVOLUTION OP THE FINS OF FISHES. 1043 



This proves conclusively tliat the diphycercal condition so perfectly 

 preserved in Ceratodus is actually recapitulated by the larvae of exist- 

 ing forms, since the primitive embryonic rays undoubtedly represen^^ 

 almost exactly the permanent rays of that form. 



Taking the examples of SipJiostoma nud Hippocampiis, both are highly 

 specialized types of Teleosts; in the former a dorsal, anal, and caudal 

 are developed directly in special discontinuous folds in the situations 

 where they are permanent, though an abnormal si)ecimen shown me 

 by Dr. T. H. Bean has a small second anal developed in an intermediate 

 situation, where it is readily conceivable that it was derived from a 

 part of the continuous tract which in other fish embryos gives rise to 

 all of the median fins. This specimen must, therefore, be regarded as an 

 example in which there has been a very imperfectly successful attempt 

 at reversion towards the most ancient and i^rimitive condition of the 

 caudal skeleton, which is permanent in Ceratodus and Profopterus. The 

 tail of Siphostoma is, however, developed in much the same way as in 

 other forms, though the dorsal lohe, or tip of the lophocercal condition 

 of the tail is very feebly marked. 



Hippocampus, unlike the last-named genus, has passed through a still 

 more extreme series of modifications, and in consequence not only de- 

 velops its median fins in separate or discontinuous folds, but the tail, 

 in consequence of the acquisition of a prehensile function, has also 

 almost lost all trace of a median caudal fold, this fin altogether failing 

 to develop at any subsequent stage. Fig. 3, PI. IV. With other types this 

 form contrasts in the most remarkable way, since it is the only form in 

 which the cartilaginous axis of the body is bent and coiled upon itself 

 ventrally, or in a direction just the reverse of the usual one in the larvae 

 of fishes. From the remarkable manner iti which this fish swims, with 

 the body in a nearly erect position, and the use of the tail as a grasp- 

 ing organ, it is fair to assume that this part of the animal has suffered 

 degeneration as far as the absence of a caudal fin is concerned, and 

 specialization, in consequence of its development into a grasping organ, 

 which has involved the modification of the caudal muscular somites 

 into special flexor muscles having a ventral position. 



In (?aw&?ma the failure to develop a continuous median fold is rather 

 remarkable. It is possible, in consequence of its viviparous develop- 

 ment, that the development of the median fins has been abbreviated 

 and followed a more direct path, as just noticed in the LoplioorancMl. 

 Since the young of Gambusia are developed within the body of the parent 

 female till they are nearly like the adult in form, it may be that the 

 usual method of development of the fins has given place to a much more 

 direct one, influenced possibly by inclosure within the ovary of the parent, 

 as a result of which hereditary influences would be intensified. At any 

 rate, it is certain that the condition of development reached at the time 

 of the first appearance of the median fins as separate folds is no more 

 advanced in other respects than in an embryo Salmon or Shad, in which 

 there is a continuous median flu-fold still present. 



