vi EXTERNAL CHARACTERS l6l 



Diplurus, and Undina (Fig. 278), there is evidence that the latter 

 modification has actually taken place, for the atrophying terminal 

 part of the tail, with a vestige of the original caudal fin, is still 

 retained as an axial prolongation between and even beyond the 

 secondarily formed caudal fin. To this secondary diphycercal tail 

 the term " gephyrocercal " has been applied. The apparent diphy- 

 cercal tail of many Fishes, and especially of Teleosts, is really a 

 gephyrocercal structure. The ancestral evolution of the different 

 types of caudal fin is recapitulated in the embryonic histories of 

 their possessors. The heterocercal condition of an adult Fish is 

 always preceded by a transitory embryonic diphycercal stage : 

 from the same starting - point the homocercal condition is 

 attained after passing through a heterocercal stage ; while the 

 gephyrocercal may perhaps be derived by degeneration from any 

 one of the others. 



The normal function of the fins, both median and paired, has 

 reference to locomotion in the form of progression, steering or 

 balancing, but in not a few Fishes the fins may be variously 

 modified and adapted for quite different purposes ; and especially 

 is this the case in the dominant group of existing Fishes the 

 Teleostei. Thus, to quote a few examples, the first dorsal fin of 

 the Sucker-Fishes (Hemora, Eclieneis) forms a cephalic sucker, 

 by means of which the Fish attaches itself to Sharks and 

 Turtles (Fig. 421); or, as in the Angler-Fish (Lophius), its 

 anterior rays are much elongated, and terminate in lobes which 

 serve as a bait to attract the prey on which the animal feeds ; 

 again, in some of the deep-sea Fishes the dorsal fin, like the 

 pectoral and caudal fins in others of a similar habitat, is pro- 

 duced into long trailing filaments whose use is probably tactile. 

 The pelagic young of many Teleosts, such as some of the 

 Eibbon- Fishes and the Horse -Mackerels (Caranx), also have 

 certain of their fin-rays prolonged into similar filaments. 

 The pectoral fins are enormously elongated and wing-like in 

 the Flying-Fishes (Uxocoetus), and, after the fashion of a para- 

 chute, serve to sustain the Fish in its flying leaps through the 

 air. They are also similarly modified for a like purpose in the 

 so-called Flying-Gurnard (Dactylopterus volitans). The pectoral 

 fins may also be used for progression on land, as in the African 

 and East Indian Goby (Perioplithalmus), where the fins are large 

 and muscular and are applied to the ground like feet, enabling 



VOL. VII M 



