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MANUAL OF ZOOLOGY. 



they occupy the middle line of the body, and are not sym- 

 metrically disposed in pairs. Most commonly, the median 

 fins consist of one or two expansions of the dorsal integument, 

 called the "dorsal fins" (fig. 166, d, d'} ; one or two on the 



Fig. 166. Outline of a fish (Perca granulata), showing the paired and unpaired fins. 

 p One of the pectoral fins ; ?' One of the ventral fins ; d First dorsal fin ; d' Second 

 dorsal fin ; a Anal fin ; c Caudal fin. 



ventral surface near the anus the "anal fins " (fig. 166, a) ; and 

 a broad fin at the extremity 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 previ- 

 ously 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 Humphrey, are 

 truly to be regarded as formed by 

 the coalescence of two lateral ele- 

 ments in the mesial plane of the 

 body. 



The caudal fin or tail of fishes 

 is always set vertically at the ex- 

 tremity 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 

 differs altogether from the hori- 

 zontal integumentary expansion which constitutes the tail of 

 the Whales, Dolphins, and Sirenia (Dugong and Manatee)., In 



Fig. 167. Tails of different fishes. 

 a Homocercal tail (Sword-fish) ; 

 b Heterocercal tail (Sturgeon). 



