VI EXTERNAL CHARACTERS 161 
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- 
cereal 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-poimt 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 
Telecstei. Thus, to quote a few examples, the first dorsal fin of 
the Sucker-Fishes (Remora, Echeneis) 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 
Ribbon-Fishes and the Horse-Mackerels (Caranz), also have 
certain of their fin-rays prolonged into similar filaments. 
The pectoral fins are enormously elongated and wing-like in 
the Flying-Fishes (Zxocoetus), 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 (Periophthalmus), where the fins are large 
and muscular and are applied to the ground like feet, enabling 
WO Ter VeLL M 
