558 



CHORD AT A. 



the l pearl organs/ Enormous numbers of large slime cells give 

 the fishes their well-known slippery skins. Since the epidermis 

 contributes nothing to the firmness of the body walls, all protective 

 structures arise from the derma, which is composed of many layers 

 of dense connective tissue and furnishes the characteristic dermal 

 skeleton, the scales. These lie at the boundary of epidermis and 

 derma, commonly imbedded in pockets of the latter, and are, on 

 account of their different structure, of systematic value, although 

 the classification based entirely upon them is no longer retained. 

 The placoid scales (fig. 554, 587, 4) nave already been men- 

 tioned, because they form the 

 starting point for dermal ossifica- 

 tions and teeth (p. 515). They 

 are rhombic bony plates, usually 

 close together like a mosaic, but 

 not overlapping. In the centre 

 of each is a spine, directed back- 

 wards, in which is a pulp cavity, 

 while the tip of the spine is cov- 

 ered with a cap of hard substance, 

 variously called enamel or vitro- 

 dentine. 



The ganoid scales (fig. 587, 

 3) are usually rhomboid and 

 arranged like parquetry. In the 

 early stages they may bear teeth, 

 The outer surface is always covered 

 with a thick layer of 'ganoin/ which gives, even in fossils, an 

 iridescent effect, a most characteristic feature. The ganoin is no 

 longer regarded as enamel, but the most superficial layer of dentine 

 (vitrodentine). 



Cycloid and ctenoid scales are closely related. They are always 

 more loosely placed in the pockets, from which they are easily with- 

 drawn as in ' scaling ' a fish. They are arranged in oblique, trans- 

 verse, and longitudinal rows, and overlap like shingles, one scale 

 covering the parts of two scales behind. The cycloid scales (fig. 

 587, 1) are approximately circular with a middle point, surrounded 

 by concentric lines, from which go radiating lines. The ctenoid 

 scale (2} has the radial and concentric lines of the cycloid, but has 

 the hinder edge truncate and the free portion bearing small spines 

 or teeth, processes of the concentric ridges. 



Besides these types of scales many fishes bear considerable 



FIG. 587. Scales of fishes. 1, cycloid; 

 ctenoid; 3, ganoid; A, placoid. 



but these are lost in the adult. 



