4'J4 



CHORDATA. 



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

 bounJarv of epidermis and corium, con\monly iml>cddeil in pockets of the 

 latter, and are, on account of their dilferent structure, of systematic value, 

 ahhou_L:;h the classification liased entirely upon them is no longer retained. 

 The placoid scales Uli^^- 51°- 545. 4) h^^^'^" already l>een mentioned, 

 because thev form the starting point for dermal ossifications and teeth 

 (p, 4ii)- Thev are rhomliic f>ony plates, usually close together like a 

 mosaic, luit 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, xari- 

 ouslv called iiuimcl or vilrodciiliiic. 



The ganoid scales (3) are usually 

 rhomljoid and arranged like par- 

 quetry. In the early stages they may 

 bear teeth, liut these are lost in the 

 adult. The outer surface is always 

 covered with a thick layer of ga)ioiii, 

 which gixes, even iir fossils, an iri- 

 descent effect, a most characteristic 

 feature. The gairoin is no longer 

 regarded as enamel, liut the most 

 superlicial layer of dentine (;vilro- 



dci!li)!c). 



Cvcloid and ctenoid scales are closelv related. They are always 

 more looselv placed in the pockets, from which they are easily withdrawn 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 liehind. The cycloid scales (i) are a]iproximately circular, 

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

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

 centric lines of the cvcloid, luit has the hinder edge truncate atrd the free 

 portion bearing small spines or teeth, processes of the coitceritric ridges. 



Besides these types of scales maity fishes bear considerable spines 

 (stroirglv de\'eloped single scales) and larger l)ony pilates, these last usually 

 resulting from the fusion of numerous scales. 



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

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

 cardial walls. In some fishes [Alhiirmis hiridiis) from their iridescence these 

 crystals become of commercial vahie, forming tlie important jiart of essence of 

 pearl, used in making artilieial |)earls. The otlier colors of fishes are due in 

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



Fig. 545. — Scales of fislies. i, 

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



