226 



AN INTRODUCTION TO ZOOLOGY 



polygonal plates. To these are attached a large number of dermo- 

 trichia, giving the characteristic outline to the fin. 



The pelvic girdle is a more simple structure, and consists of one 

 more or less straight bar of cartilage running transversely in the 

 muscles of the body wall, immediately in front of the cloaca. Its 



postero-lateral corner is hol- 

 lowed to form an articular 

 facet that bears the pelvic 

 fin, while antero-laterally it 

 is produced forward in a 

 short bluntly pointed process. 

 The fin skeleton is also 

 simpler, the somactidia hav- 

 ing fused to form a single 

 basalium, termed the basi- 

 pterygium. This bears a 

 series of radialia, terminating 

 in polygonal plates, and these 

 in turn carry the customary 

 dermotrichia. In the male 

 fish the innermost radialium 

 is greatly enlarged, grooved 

 on its dorsal surface and 

 constitutes the skeleton of 

 the clasper. 



We have seen, then, 

 that the structure of all the 

 fins is essentially the same, they are indeed homologous. Furthermore, 

 it is fairly clear that the median fins have been evolved from one 

 long continuous fin running along the back, around the tail and 

 forward ventrally to the anus, for we find such a fin not only in such 

 forms as Amphioxus, and the lowly Cyclostomes, but also in the 

 embryos of some fish and in the tadpole. This long fin has been 

 split up probably in response to the needs of stability and locomotion, 

 and has been developed at the points of the greatest mechanical 

 advantage, while the intervening portions have been suppressed. 

 The origin of the paired fins is not so obvious, but is a matter of 

 interest. A theory known as " the lateral fold theory " is perhaps 

 the most generally held. According to this the ancestral fish form 

 had a lateral fold of skin running forward from the cloaca to the 

 region of the gill clefts, somewhat similar to that found in Amphioxus. 

 Like the median fin, these lateral folds were developed at some points 

 and suppressed over the remainder. A fair amount of evidence 

 supports this point of view. Thus in Cladoselache, a fossil shark 



FIG. 



72. Pelvic girdle and fin of 

 Scy Ilium. 



B., basipter 

 dermolrichia ; 

 bar ; P.P., pelvic fin 



ium ; C., skeleton of clasper ; D., 

 polygonal plates; P.B., pelvic 

 n ; R., radialia. 



