318 SYSTEMATIC ZOOLOGY. 



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

 ter extending over them. These scales may be of four 

 kinds, the placoid, ctenoid, cycloid, and ganoid. Placoid 

 scales are hard plates, in structure much like teeth, with 

 usually a spine projecting backwards from the surface. 

 Cycloid scales are much softer, more or less circular in 

 outline, and with no projecting spine. Ctenoid scales differ 

 from cycloid in having the free edge of the scale toothed 

 somewhat like a comb. Ganoid scales are either rhomboid 

 or circular in outline and are covered externally with a 

 peculiar enamel layer. At one time fishes were classified 

 according to the scales, but this was found to be unnatural. 

 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 girdles (pectoral in front, pelvic 

 behind), which extend across 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 and of directing the body 

 upwards or downwards ; the caudal is the chief swimming- 

 organ. The caudal fin presents three interesting con- 

 ditions. In all fishes it is at first diphycercal (fig. 120, A); 

 that is, the vertebral column runs out in a straight line, 

 dividing the fin into equal and symmetrical halves. 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 (fig. 120, B}. 

 This condition is permanent in the Selachii and most 

 ganoids, but in the bony fishes the lower lobe grows out 

 equal to the other, and the tail becomes homocercal, 



