104 



VERTEBRATE ZOOLOGY 



specialized groups these fins may become greatly exaggerated as in 

 Zanclus (Fig. 86) and in Dendrochirus (Fig. 90). 



The most significant evolutionary processes concern the caudal fin 

 and in this region of the fish's body we have changes that parallel 

 those that have occurred in the caudal regions of various land animals, 

 involving, on the one hand, more or less pronounced foreshortening 

 or atrophy, and, on the other hand, excessive prolongation or hyper- 

 trophy of the tail region. Beginning with what appears to be the 



FIG. 55. Diagram of the origin of the median and paired appendages from 

 lateral fin-folds. The arrows indicate the points of junction, dorsal and ventral, 

 of the paired fin-folds with the median fin-folds. (Modified after Kingsley.) 



ancestral condition, we have a type of caudal fin called diphycercal 

 (Fig. 56, A), which is evenly developed both above and below the 

 notochord of the tail. The supporting rays above the notochord 

 (epichordal rays) are as well developed as those below the notochord 

 (hypochordal rays). A type of caudal fin that appears to be next in 

 order of antiquity is the heterocercal type (Fig. 56, B), in which the 

 epichordal rays, and consequently the upper lobe of the fin, are 

 much less strongly developed than the hypochordals. This type of 

 fin is found in most elasmobranchs, in the Holocephali, in the 

 Chondrostei and, in a modified form, in the Holostei. A third type, 



