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 yertebral 



Fig. 18. 



Di pliycercal caudal fin ofPolyjjterus bichir, Bonai). Receiit. 

 eh, Minder eud of vertebral column (after KöUiker). 



Externally and internally heterocercal caudal 

 fin of the Sturgeon (Acipenser). 



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 



Fi(i. 2(1. 



Internally lieterocercal caudal ttn of Amia. 

 Recent. ch, Cartilaginous hinder end of the ver- 

 tebral column, with rudlnient of notochord (after 

 Kölliker). 



Fig. 21. 



Externally hoinocercal (stegurous) caudal fin of the 

 SalniOH (Salmo salar). a, b, c, Upper roofing bones of 

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

 notochord (after Kölliker). 



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. 



