PISCES (FISHES). 



The forms to which the name Fishes is usually applied 

 have a body adapted in shape and structure for an aquatic 

 life. It is usually covered with scales, which lie between 

 the two layers (corium and epidermis) of the skin, the lat- 

 ter extending over them. These scales may be of four 

 kinds, the placoid, ctenoid, and cycloid already mentioned 

 (pp. 10 and 17), and the ganoid, either rhomboid or circu- 

 lar in outline, and covered externally with a peculiar 

 enamel layer. 



The fins are adapted to fanning the water, being broad 

 plates with an internal stiffening skeleton. Usually both 

 anterior and posterior paired fins are present, and these 

 are supported on skeletal arches or girdles (pectoral in 

 front, pelvic behind), which extend around the body 

 beneath, but which have no connection with the vertebral 

 column, nor with any structure like a breast-bone. The 

 pectoral, however, is frequently joined to the skull. The 

 paired fins are largely organs of balancing; the caudal is 

 the chief swimming organ. The caudal fin presents three 

 interesting conditions. In all fishes it is at first diphy- 

 cercal; that is, the vertebral column runs out in a straight 

 line, dividing the fin into equal and symmetrical lobes. 

 This condition is retained in a few forms. In others, with 

 growth, the vertebral axis becomes bent upwards, and a 

 secondary lower lobe is developed which, as it is smaller 

 than the other, gives the heterocercal condition (p. 9). 



33 



