312 VERTEBRATA. 



in number, and in some cases there is but a single fringe 

 running round the posterior extremity of the 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 commonly, the median 

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

 called the "dorsal fins" (fig. 277, d d 1 ) ; one or two on the 

 ventral surface near the anus the "anal fins" (fig. 277, 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 sup- 

 port the median fins are articulated with the so-called inter- 

 spinous 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 Humphrey, are truly to be regarded 

 as formed by the coalescence of two lateral elements in the 

 mesial plane of the body. 



Fig. 277. Outline of a fish (Perca rramtlata), showing the paired and unpaired fins. 

 / One of the pectoral fins ; v One of the ventral fins ; d First dorsal fin ; d' Second dor- 

 sal fin ; a Anal fin ; c Caudal fin. 



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 differs 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 two very dis- 

 tinct types of structure, termed respectively the "homocercal " 

 and " heterocercal " type of tail (fig. 278). The homocercal 

 tail is the one which most commonly occurs in our modern 

 fishes, and it is characterised by the fact that the two lobes of 



