CLASS I 



PISCES 



extends in a straight line to the end of the trunk, and is symmetrically 

 fringed above and below with the caudal fin. The Cyclostomi: Dipnoi and 

 many Crossopterygii (Fig. 18) remain in this embryonic stage throughout life 

 In the heterocercal fishes (sharks, rays, many ganoids) the end of the vertebral 



Fig. 18. 



Fig. 19. 



column is bent upwards, and completely passes into the upper lobe of the 

 caudal fin, which is usually elongated, and exceeds the lower lobe in size (Fig. 

 19) Between the diphycercal and heterocercal types of tail there are many 

 gradations. In the hetero-diphycercal tail the vertebral column is elongated 

 in a gentle upward curve at its hinder end, and fringed above and below with 



Fig. 20. 



Internally heterocercal caudal tin of Amia 

 Recent, ch, Cartilaginous hinder end of the ver- 

 tebral column, with rudiment of notochord (after 

 Kolliker). 



Externally homocercal (stegurous) caudal tin of the 

 Salmon (Salmo salar). <<, b, c, Upper roofing bones of 

 llic end of the notochord; ch, Projecting end of the 

 notochord (after Kolliker). 



fin rays ; but the rays of the upper lobe are much less developed than those 

 of the lower lobe. Sometimes also the external caudal fin consists of two 

 similar lobes, while the vertebral column is bent upwards, and extends far 

 into the upper lobe (Fig. 20). Such fins are externally homocercal, internally 

 heterocercal. In the hemi-heterocercal tail the upper lobe of the externally 

 symmetrical caudal fin is still partly or completely covered with scales. 



