APPENDICULAR SKELETON 137 



in cartilage (Teleostomes) : recent researches indicate that the 

 latter are ectodermal in origin. 1 



Median fins are also present in the Amphibia, in which they 

 may persist throughout life (e.g. Perennibranchiata), or only occur 

 in the larval stage ; occasionally also they become specially devel- 

 oped during the breeding season (e.g. Newt). They have the 

 form of a continuous integumentary fold extending round the tail 

 and along the back for a greater or less distance, but enclose no 

 skeletal elements. 



Amongst Reptiles one or more median fins were present in 

 Ichthyosaurus, and these are comparable to the dorsal fins occur- 

 ring in the Cetacea amongst Mammals : in both cases they, like 

 the horizontal tail fin of these forms, must be looked upon as 

 structures acquired secondarily in connection with an aquatic 

 existence. 



B. Paired Fins or Limbs. 



As regards the origin of the paired fins, there is much difference 

 of opinion. According to one view, they correspond to modified 

 gill-arches and rays, the former giving rise to the pectoral and 

 pelvic arches or girdles, and the latter to the free portion of each 

 fin, one of the rays becoming enlarged so that the others are 

 attached in a row on either side of it, instead of to the arch. This 

 would result in a biserial form of fin, the " archipterygium " of 

 Gegenbaur, such as is most nearly retained in Cera todus (Fig. 118), 

 and is also indicated in many Elasmobranchs. The fact that the 

 branchial arches are situated in the pharyngeal wall and the limb 

 arches in the body-wall, alone forms an important objection to this 

 theory. 



Another view, which seems to be the more likely one, is 

 as follows. It is highly probable that primitive Vertebrates at 

 one time possessed, in addition to the median fins, a pair of con- 

 tinuous lateral fin-folds, traces of which, beginning with a prolifera- 

 tion of the mesoderm, can still be recognised in young embryos of 

 Elasmobranchs (Fig. 97) and to a less extent in those of other 

 Fishes and of Amphibians, and which, though never continuous, are 

 indicated by muscle-buds on the intermediate myotomes. They 

 extended backwards along the sides of the body from just behind 

 the head, gradually converging towards the anal region, where 

 they became continuous with the ventral part of the median fin- 

 fold (Fig. 96, A), and in this respect resembled the persistent lateral 

 or metapleural folds present'in the adult Amphioxus, though it is 



1 The dermal fin rays or dtrmotrichia are classified by Goodrich as follows : 

 1. Horny ceratotrichia in Elasmobranchs; 2. Bony hptotrichia in Teleostomes; 

 3. Horny actinotrichia occurring in the embryo and in the margins of the fins of 

 adult Teleostomes, in addition to (2) ; 4. Fibrous, calcified,or horny camptotrichia 

 in Dipnoans : it is doubtful whether the last-mentioned correspond to (1) or to (2). 



