55S 



CHORD AT A. 



the ' pearl organs.' Enormous numbers of large slime cells give 

 the fishes their "ivoll-known slippery skins. Since the epidermis 

 contributes nothing to the firmness of the body walls, all j^rotective 

 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 ejiidermis and 

 derma, commonly imbedded in joockets 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. 

 Tlie placoid scales (fig. 5.54, 587, .0 have already been men- 

 tioned, because they form the 

 starting point for dermal ossifica- 

 tions and teeth (p. 51.5). They 

 are rhombic bony pilates, usually 

 close together like a mosaic, but 

 not overlap2iing. 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, 

 o) 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 sujierficial layer of dentine 

 (vitrodentine). 



Cycloid and ctenoid scales are closely related. They are always 

 more loosely jilaced in the pockets, from wliich they are easily witli- 

 drawn as in ' scaling ' a fish. The}' 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. 

 5S7, 1) are ajiproximately circular with a middle j^oint, surrounded 

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

 scale (S) has the radial and concentric lines of the cycloid, but has 

 the iiinder edge truncate and the free portion bearing small sjiines 

 or teeth, jirocesses of the concentric ridges. 



Besides these types of scales many fishes bear considerable 



Fin 



-Scales of fislies. i, cyciuid; 

 , ganoid; i, placoid. 



ctenoid 



but these are lost in the adult. 



