116 



FISHES. 



body. In all cases, however, the median fins are " azygous " 

 — that is to say, they occupy the middle line of the body, 

 and are not symmetrically disposed in pairs. Most com- 

 monly, the median fins consist of one or two expansions of 

 the dorsal integument, called the "dorsal fins" (fig. 490, 

 d, d') ; one or two on the ventral surface near the anus — 

 the "anal fins" (fig. 490, a); and a broad fin at the ex- 

 tremity of the vertebral column, called the " caudal fin " or 

 tail (c). In all cases, the rays which support the median 

 fins are articulated with the so-called interspinous bones, 

 which have been previously described. Though called 

 " median," from their position in the middle line of the body, 

 and from their being unpaired, the median fins of Fishes, as 

 shown by Goodsir and Humphrej^, are truly to be regarded 

 as formed by the coalescence of two lateral elements in the 

 mesial plane of the body. 



Fig. 490. — Outline of a fish (Perca granulata), showing the paired and unpaired fins, p. 

 One of the pectoral fins ; v, One of the ventral tins ; d, First dorsal fin ; d', Second dorsal fln ; 

 a, Anal fin ; c, Caudal fln. 



The caudal fin, or tail, of fishes is always set vertically at 

 the extremity of the spine, so as to work from side to side, 

 and it is the chief organ of progression in the fishes. In its 

 vertical position, and in the possession of fin-rays, it difters 

 altogether from the horizontal integumentary expansion 

 which constitutes the tail of the Whales, Dolphins, and 

 Sirenia (Dugong and Manatee). In the form of the tail, 

 fishes exhibit some striking differences. In some of the 

 Bony fishes and Ganoids, the caudal extremity of the spine 



