PISCES 



103 



cated around the latter and continued forward as two lateral fin-folds 

 that reached almost to the head. Two pieces of evidence support 

 this theory. The first is, that in Amphioxus the median fin-fold 

 actually does fork and continue forward on the ventral side as the 

 metapleural folds. The second is that in the primitive shark Clado- 

 selache (Fig. 65) the paired fins have rays parallel to one another much 

 as in the median fin system, and give the impression that they are 

 merely parts of a system continuous with the latter. Two diagrams 

 are here presented that illustrate the origin of the paired fins from 



BF Jn 



Fig. 54. — Fin-fold origin of paired fins. A. The hypothetical undifferentiated 

 condition. B. The manner in which it is thought that the permanent fins were 

 derived from the continuous fin-folds. AF, Anal fin; An, anus; BF, pelvic fins; 

 BrF, pectoral fins; D, continuous dorsal fin-fold; FF, posterior dorsal fin; S and 

 S, paired ventral lateral fin-folds; S'-, median ventral fin-fold continuous with 

 S and S; SF, superior or dorsal lobe of caudal fin. (From Wiedersheim.) 



lateral fin-folds. The first (Fig. 54) is that of Wiedersheim, and the 

 second is adapted from that of Kingsley (Fig. 55), involving double 

 dorsal as well as double ventral fin-folds. 



The most primitive types of fishes or fish-like creatures have the 

 median fin system unbroken and regionally unspecialized, but most 

 of the fishes proper have the continuous median fin subdivided into 

 one, two or three dorsal fins, a caudal fin, and one or two anal or ven- 

 tral fins. The degree of development of these various regional special- 

 izations of the median fin system varies greatly according to the habit 

 and degree of speciaUzation of the group. In the more generalized 

 types the size and degree of elaboration of these fins remain well 

 within the bounds of bodily symmetry, but in some of the highly 



