6o 



A HISTORY OF FISHES 



of the vertebral column is bent upwards, and the two lobes of 

 the fin, which still retain their continuity around the tip of the 

 tail, become differentiated into a small upper lobe and a much 

 larger lower lobe, the latter taking its origin entirely from the 

 lower side of the upturned backbone (Fig. 26a). In some of 

 the more generalised Bony Fishes such as the Gar Pikes {Lepi- 

 dosteus), Bow-fin (Amia), and Bichirs [Polypterus) , the upturned 

 part is so much shortened that the lower lobe of the tail-fin 



Fig. 26. STRUCTURE OF CAUDAL FINS. 



A. Sturgeon (Acipemer sp.) ; b. Ten-pounder (Elops saurus) ; c. Haddock 

 {Gadus aeglifinus) ; d. Sun-fish (Mola mola). 



comes to he at the end of the fish, and externally has the 

 appearance of a symmetrical structure. 



In most of the higher Bony Fishes {i.e. in the Salmon, Perch 

 Plaice, etc.) the tail is of the homocercal type. Outwardly such 

 a tail seems perfectly symmetrical, the prolongation of the axis 

 of the body appearing to divide the fin into equal-sized and 

 continuous upper and lower lobes. Dissection, however, reveals 

 that this superficial appearance is misleading and that this 

 type of tail is nothing more than a modified heterocercal con- 

 dition. The hinder end of the \ ertebral column turns upwards 



