446 



COLLEGE ZOOLOGY 



in certain other species they are in the throat region and are 

 said to be jugular in position. In most fishes the fins are sup- 



BF A n 



Fig. 375. — Diagram showing A, the undifferentiated condition of the paired 

 and unpaired fins in the embryo, and B, the manner in which the permanent 

 fins are formed from the continuous folds. AF, anal fin; An, anus; BF, pelvic 

 fin; BrF, pectoral fin; D, dorsal fin-fold; FF, dorsal fin; RF, dorsal fin; 

 SF, tail-fin; S, S, lateral folds which unite at S' to form ventral fold. (From 

 Wiedersheim.) 



ported, as in the perch, by cartilaginous rods and bony spines; 

 this type of appendage is called an ichthyo pterygium. In a few 

 fishes {e.g. Polypterus, Fig. 380) the pectoral fins have a median 



axis, which may be jointed, 

 and bears rays about the 

 edge ; this is termed an 

 archipterygium (Fig. 376). 

 The fingered appendage 

 {cheiro pterygium) of higher 

 vertebrates may have arisen 

 from the latter type. 



The shape of the caudal 

 fin and the terminal por- 



Fig. 376. — Archipterygial pectoral fin tion of the tail differs in 



n a lun , g - fis p h ' ^Tl^Z rf' ba ff 1; the main groups of fishes, 



D, dermal; R, radial. (From Dean, after . . 



Howes.) and is therefore of im- 



