CLASS OSTEICHTHYES. BONY FISHES 



387 



Dorsal fin f o d 



Ventral fold 



Lateral folds 



Anus 

 Dorsal finsn 



B 



Tail fin 



Pectoral fin 



Pelvic fin Anus Anal fin 



Figure 259. Diagrams illustrating the Enfold theory of the origin of fins. A, the continuous 

 folds of the paired and unpaired fins in the embryo. B, parts of the continuous folds disappear 

 to form the permanent fins. (After Wiedersheim. ) 



pelvic, and pectoral fins persist. The pelvic 

 fins vary considerably in position. In the 

 perch (Fig. 253) they are situated beneath 

 the pectoral fins; in the fresh-water catfish, 

 they are just in front of the anus and are 

 called abdominal; and in certain other 

 species they are in the throat region. 



The shape of the caudal fin and the 

 terminal portion of the tail differs in the 

 main groups of fishes and is therefore of 

 importance in classification (Fig. 260). 



The three main types of caudal or tail fins 

 found in fishes are: heterocercal, diphycercal, 

 and homocercal. The heterocercal tail is 

 found in modern sharks; it is two-lobed, 

 with the vertebral column extending into the 

 larger dorsal lobe. The stroke of the asym- 



metrical heterocercal tail forces the anterior 

 part of the body downward. This type is 

 therefore of advantage to and characteristic 

 of those fishes that have ventrally situated 

 mouths and feed on the bottom. 



In the diphycercal type the vertebral col- 

 umn extends straight back to the tip of the 

 body, with the tail fin developed symmetric- 

 ally above and below it; the living lungfishes 

 have tail fins of this type. The homocercal 

 fin is externally symmetrical, but the internal 

 structure shows that the backbone extends 

 into the dorsal lobe. The stroke of the homo- 

 cercal tail forces the fish straight forward. 

 It is characteristic of those fishes with a 

 terminal mouth and is the type possessed by 

 most bony fishes. 



Heterocercal Diphycercal Homocercal 



Figure 260. Types of tails in fishes. 



