File:A manual of zoology (1912) (20768778582).jpg

Title: A manual of zoology
Identifier: cu31924031530540 (find matches)
Year: 1912 (1910s)
Authors: Hertwig, Richard, 1850-1937; Kingsley, J. S. (John Sterling), 1854-1929, ed. and tr
Subjects: Zoology
Publisher: New York, H. Holt
Contributing Library: Cornell University Library
Digitizing Sponsor: MSN

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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 They are always more looselv placed in the pockets, from which they are easily withdrawn as in 'scaling' a fish.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
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Fig. 545.—Scales of fislies. i, cloid; J, ctenoid; 3, ganoid; 4, placoid.

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