50 ZOOLOGY. 



body, and are, for this reason, termed median fins. 

 These fins are joined to a series of pointed bones called 

 interspinous bones, which are connected at their inner 

 ends by ligaments with the spinous processes of the 

 vertebrae. The median fins on the back, one or two in 

 number, are called dorsal (Lat. dorsum, the back) fins; 



it' 



FIG. 17. SKELETON OF THE PERCH. 



/, operculum; gg f , vertebral column; h, one of the pectoral fins; i t ventral 

 fin ; kl, first and second dorsal fins, supported OD interspinpus bones; m anal 

 fin, with its interspinous bones . 



those near the vent, anal fins; and the fin at the ex- 

 tremity of the vertebral column is termed the caudal 

 (Lat. cauda, the tail) or tail fin. The median fins of 

 fishes differ from those that are met with in the larvae of 

 the Amphibia, in being supported by " fin rays." The 

 caudal fin, which is the principal agent in locomotion, is 

 always placed vertically, and not horizontally as in the 

 whale tribe. 



There are two types of caudal fin, homocercal (Gr. 

 homos, same; herlcos, a tail), and heterocercal (Gr. heteros, 

 different; Jcerkos, a tail). In the homocercal type, the ver- 

 tebral column terminates at the commencement of the tail 

 lobes, which are of the same shape and of equal size, In 



