494 



CHORDATA. 



furnishes the characteristic dermal skeleton, the scales. These lie at the 

 boundary of epidermis and corium, 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 (figs. 510, 545, 4) have already been mentioned, 

 because they form the starting point for dermal ossifications and teeth 

 (p. 451). They are rhombic bony plates, usually close together like a 

 mosaic, but not overlapping. In the centre of each is a spine, directed 



backwards, in which is a pulp cavity, 

 while the tip of the spine is covered 

 with a cap of hard substance, vari- 

 ously called enamel or vitrodentine. 



The ganoid scales (3) are usually 

 rhomboid and arranged like par- 

 quetry. In the early stages they may 

 bear teeth, but these are lost in the 

 adult. The outer surface is always 

 covered with a thick layer of ganoin, 

 which gives, even in fossils, an iri- 

 descent effect, a most characteristic 

 feature. The ganoin is no longer 

 regarded as enamel, but the most 

 superficial layer of dentine (vitro- 

 dentine) . 



Cycloid and ctenoid scales are closely related. They are always 

 more loosely placed in the pockets, from which they are easily withdrawn 

 as in 'scaling' a fish. They are arranged in oblique, transverse, and 

 longitudinal rows, and overlap like shingles, one scale covering the parts 

 of two scales behind. The cycloid scales (i) arc approximately circular, 

 marked on one side by concentric lines, while on the other numerous lines 

 radiate to the periphery. The ctenoid scale (2) has the radial and con- 

 centric 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 spines 

 (strongly developed single scales) and larger bony plates, these last usually 

 resulting from the fusion of numerous scales. 



The coloration of fishes may have three sources. The silvery lustre is due 

 to crystals of guanin which occur in the skin and in the peritoneum and peri- 

 cardial walls. In SOUK- fishes (Alburnits lucidus) from their iridescence these 

 crystals become of commercial value, forming the important part of essence of 

 pearl, used in making artificial pearls. The other colors of fishes are due in 

 part to the numerous strongly pigmented fat celis, in part to chromatophores in 



FIG. 545. Scales of fishes, i, cy- 

 cloid; 2, ctenoid; 3, ganoid; 4, placoid. 



