THE MICROSCOPE IN ZOOLOGY. 85 



are developed in the substance of the true skin ; but 

 those of reptiles, the feathers of birds, the hairs, nails, 

 claws, and horns of mammalia, are developed, not 

 within, but upon the surface of the true skin. The 

 scales of fish are either ctenoid, i.e., furnished at their 

 posterior extremities with comb-like teeth, as the 

 scales of the sole ; cycloid, having scales more or less 

 round, as in the salmon, roach, herring, &c. ; ganoid 

 (from a Greek word ganos, " splendour "), having scales 

 whose substance is essentially bony, hard, and highly 

 polished (this kind has few existing representatives, 

 but numbers are found as fossils) ; or placoid, i.e., 

 having scales separately embedded in the skin, and 

 projecting from its surface in various forms. "In 

 studying the structure of the more highly developed 

 scales, we may take as an illustration that of the carp, 

 in which two very distinct layers can be made out by 

 a vertical section, with a third but incomplete layer 

 interposed between them. The outer layer is com- 

 posed of several concentric laminae of a structureless 

 transparent substance, like that of cartilage ; the outer- 

 most of these laminae is the smallest, and the size of 

 the plates increases progressively from without in- 

 wards, so that their margins appear on the surface as 

 a series of concentric lines, and their surfaces are 

 thrown into ridges and furrows which commonly have 

 a radiating direction. The inner layer is composed of 

 numerous laminae of a fibrous structure, the fibres of 

 each laminae being inclined at various angles to those 

 of the laminae above and below it. Between these 

 two layers is interposed a stratum of calcareous con- 

 cretions, resembling those of the skin of the eel; 

 these are sometimes globular or spheroidal, but more 

 commonly 'lenticula/ that is, having the form of a 

 double-convex lens."* The scales of the eel are con- 



* Dr. Carpenter, page 702. 



