562 CHORDATA. 



The opercular apparatus does not occur in all fishes. It is a 

 number of bony plates and processes which arise from the hyoid 

 arch and extend backwards over the gills, protecting them. It 

 arises in part (opercular bones (9, Pro, So, lo, fig. 589) from 

 the hyomandibular, in part (branchostegal rays) from the hyoid 

 bone. The significance of this apparatus will be spoken of in con- 

 nexion with the gills; it gives the fish head a definite character, 

 but covers its structure, on which account it, like the infraorbital 

 ring, is shown in red in the figure 589. 



The appendages are also influenced by the aquatic life. In 

 contrast to the cyclostomes, there are two pairs of paired fins, the 

 thoracic or pectoral, and the pelvic, ventral, or abdominal fins; in 

 contrast with Amphibia, reptiles, and mammals, which occasionally 

 have fin-like structures, the fishes have three unpaired fins, dorsal, 

 caudal, and anal fins. Only rarely, as in the eels, the ventral fins 

 are lacking; more rarely (Maraenidae) the pectorals are lost. The 

 function of the fins in swimming and in balancing makes it neces- 

 sary that they be broad and well-supported plates. Hence it is 

 that numerous skeletal parts are present; besides those preformed 

 in cartilage, numerous horny or bony rays; further, that all parts 

 should be similar and closely, even if flexibly, bound to each other. 

 Joints are unnecessary except at the base where the fins join the 

 supports and move upon the body. The supports of the paired 

 fins are the girdles, arched skeletal parts, which in the sharks are 

 held only by muscles, a statement which is true for the pelvic 

 girdle of all fishes. This is why the ventral fins so readily change 

 their place. Their primitive position is at the hinder end of the 

 body cavity (Pisces abdominales, figs. 598, 601). From this point 

 they can move forward to beneath the pectorals (Pisces thoracici, 

 fig. 602), or may even come to lie in front of them (Pisces jugu- 

 lares) in the throat region (fig. 606). The pectoral arch is united 

 to the vertebral column in the skates; to the skull by a series of 

 bones in the teleosts. 



The dorsal and anal fins are supported by elements preformed 

 in cartilages which rest upon the neural or haemal spines and in 

 turn support the fin rays. In the caudal fin the rays rest directly 

 upon the spinous processes. Three types of caudal fin are rec- 

 ognized diphycercal, heterocercal, and homocercal (fig. 10), 

 distinctions of great importance. The primitive type is the diphy- 

 cercal, in which the vertebral column extends directly into the 

 middle of the fin, dividing it into symmetrical halves. In the 

 heterocercal type the vertebral axis binds slightly upwards at the 



