THE TAIL 325 



(usually forked) appearance. Internally, however, the skeleton 

 reveals the fact that this homocercal type of tail is derived from 

 the heterocercal, and the axis can be seen to bend up at the 

 tip. It is found also that during development, the homocercal 

 tail passes through a heterocercal stage. 



In other forms the tail tapers symmetrically to a point, and 

 so comes to resemble the diphy cereal type. This secondarily 

 simplified type of tail (shown by the eel, for example) is called 

 gephyrocercal, and is the result of reduction from the hetero- 

 cercal or homocercal condition. The tail-fin of Gadus is 



Fig. 165. — Skeleton of the tail of the salmon, showing the homocercal 

 pattern of tail-fin characteristic of most Teleost fish. 



Note the up -turned vertebral column, h, hypurals ; /, lepidotrichia ; 

 v t vertebra. 



peculiar, for it is formed from the hind portions of the median 

 dorsal and ventral fins. Such a tail is called pseudocaudal. 



In Ceratodus, the tail seems to be diphycercal (and there- 

 fore primitive), because its ventral lobe is supported by separate 

 radials, and not by hypurals. There is, however, doubt about 

 this, because many of the fossil Dipnoi had heterocercal tails, 

 and if it can be proved that Ceratodus is descended from them, 

 the structure of its tail must be gephyrocercal. 



In amphibia, the tail-fins are present in the larval stages, 

 which live in water ; but they disappear when the animals 

 come out on land, to grow again in some during the water 

 sojourn of the breeding season. In the Anura (frogs and toads) 



