40 



FISHES. 



Fins. The Fins are divided into vertical or unpaired, and into 

 horizontal or paired fins. Any of them may be present or 

 absent; and their position, number, and form are most im- 

 portant guides in determining the affinities of fishes. 



The vertical fins are situated in the median dorsal line, 

 from the head to the extremity of the tail, and in the ventral 

 line of the tail. In fishes ia which they are least developed 

 or most embryonic, the vertical fin appears as a simple fold of 

 the skin surrounding the extremity of the tail. In its further 

 progress of development in the series of fishes, it gradually ex- 

 tends more forwards, and may reach even the head and vent. 

 Even in this embryonic condition the fin is generally supported 

 by fine rays, which are the continuations of, or articulated to, 

 other stronger rays supported by the processes or apophyses 

 of the vertebral column. This form of the vertical fin is very 

 common, for instance in the Eels, many Gadoid, Blennioid 

 and Ganoid fishes in which, besides, the rays have ceased to 

 be simple rods, showing more or less 

 numerous joints (simple articulated rays ; 

 Fig. 3). Branched rays are dichotomically 

 split, the joints increasing ia number to- 

 wards the extremity. 



The continuity of the vertical fin, 

 however, is interrupted in the majority of 

 fishes;, and three fins then are distin- 

 guished : one in the dorsal line — the dorsal 

 fin ; one in the ventral line behind the 

 anus — the anal fin ; and one confined to 

 the extremity of the tail — the caudal fin. 



The caudal fin is rarely symmetrical, 

 so that its upper half would be equal to 

 its lower; the greatest degree of asym- 

 metry obtains in fishes with heterocercal termination of the 

 vertebral column (see subsequently, Figs. 31, 41). In fishes in 



Fig. 3. 



1. Simple ray. 



2. Spine. 



3. Simple articulated 



ray (soft). 



4. Branched ray (soft). 



