348 



MANUAL OF ZOOLOGY. 



" 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. 



Fig. 129. Outline of a fish (Perca granulata), showing the paired and unpaired fins. 

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

 dorsal 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 distinct types 

 of structure, termed respectively 

 the "homocercal" and " hetero- 

 cercal" type of tail (fig. 130). 

 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 the tail are equal, 

 and the vertebral column, instead 

 of being prolonged into the upper 

 Fig. 130. Tails of different fishes, lobe of the tail, stops short at 



a. Homocercal tail (Sword -nsh); -L T 1- -L i i -i 



b Heterocercai tail (Sturgeon). its base. In the hetcrocercal tail, 

 on the other hand, the vertebral 



column is prolonged into the upper lobe of the tail, so that the 

 tail becomes unequally lobed, its greater portion being placed 



