CAUDAL FIN. 55 



They are branched, jointed bony rays, and appear to be merely modified 

 lepidotrichia. They are more numerous than the somactids. 



In all fishes the ventral part of the caudal differs from the 

 other median fins in the fact that the dermotrichia (fin-rays) 

 are supported directly by the haemal arches. These are fre- 

 quently imperfectly segmented from one another, and may, 

 in the adult, have the form of two or three bony plates, or even 

 of a single plate. They are sometimes called, when ossified, the 

 hypural bones. 



A few Teleostei (eel-like forms, some Gadidae, etc.) and the Dipnoi 

 constitute apparent exceptions to this rule as to the structure of the 

 caudal fin, but in the former of these it is probable that the anal fin 

 has fused with the ventral part of the caudal fin, for in a small por- 

 tion of the ventro-caudal fin a few dermotrichia are carried directly by 

 haemal arches. In the Dipnoi on the other hand, and in some Teleostei, 

 the caudal fin appears to be entirely unrepresented, for the tail gradually 

 tapers to its termination. What appears at first sight to be the ventro- 

 caudal fin is really the anal fin, and in no part of it are the dermotrichia 

 supported directly by the haemal arches. In the crossopterygian Ganoids, 

 in which there is a diphycercal tail, the ventral dermotrichia of the caudal 

 fin are clearly supported by haemal arches and not by somactids. 



Considerable importance has been attached to the form of the 

 tail and to the structure of the caudal fin in fishes. In the 

 simplest cases the vertebral column is continued straight to 

 its termination, and the dorsal and ventral part of the caudal 

 fin are equal and symmetrical with each other. This type of 

 caudal fin is called diphycercal or protocercal. In many fishes, 

 however, the posterior part of the vertebral column is bent 

 dorsalwards, and a special enlargement of the ventro-caudal 

 fin is formed at a short distance from the end of the tail. This 

 type of caudal fin is called heterocercal (Fig. 64), and is character- 

 istic of Elasmobranchs and chondrostean Ganoids. In such fishes 

 the tail may be said to be bifid, presenting a dorsal lobe and a 

 ventral lobe. The dorsal lobe consists of the real hind end of 

 the tail with the dorso-caudal (if present at all) and part of the 

 ventro-caudal fin, while the ventral lobe is the specially enlarged 

 part of the ventro-caudal fin above described. 



In Teleostei, and bony Ganoids, and some Selachians, the 

 dorsal lobe thus defined shrinks and almost disappears relatively 

 to the greatly enlarged ventral lobe, which now forms the whole 

 of the tail fin, and becomes svmmetrical in itself. Such a tail 



