SCALES OF FISHES 



1027 



beneath it, or by tearing off the entire thickness of the skin and 

 looking for them near its under surface. This is the case, for 

 example, with the common eel, and with the viviparous blenny ; of 

 either of which fish the skin is a very interesting object when dried 

 and mounted in Canada balsam, the scales being seen imbedded in 

 its substance, whilst its outer surface is studded with pigment-cells. 

 Generally speaking, however, the posterior extremity of each scale 

 projects obliquely from the general surface, carrying before it the 

 thin membrane that incloses it, which is studded with pigment- 

 cells ; and a portion of the skin ofj, almost any fish, but especially of 

 such as have scales of the ctenoid kind (that is, furnished at their 

 posterior extremities with comb-like teeth, fig. 759), when dried 

 with its scales in sit if., is a very beautiful opaque object for the low 

 powers of the microscope (fig. 758), especially with the binocular 

 arrangement. Care must be taken, however, that the light is made 

 to glance upon it in the most advan- 

 tageous manner, since the brilliance with 

 which it is reflected from the comb-like 

 projections entirely depends upon the 

 angle at which it falls upon them. The 

 only appearance of structure exhibited by 

 the thin flat scale of the eel, when ex- 

 amined microscopically, is the presence of 

 a layer of isolated spheroidal transparent 

 bodies, imbedded in a plate of like trans- 

 parence ; these, from the researches of 

 Professor W. C. Williamson l upon other 

 scales, appear not to be cells (as they 

 might readily be supposed to be), but con- 

 cretions of carbonate of lime. When the 

 scale of the eel is examined by polarised 

 light its surface exhibits a beautiful St. 



Andrew's cross ; and if a plate of selenite 



T i i i i -, i FIG. 759. Scale of sole, viewed 



is placed behind it, and the analysing as a transparent object, 

 prism be made to revolve, a remarkable 

 play of colours is presented, 



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 

 composed of several concentric laminae of a structureless trans- 

 parent substance like that of cartilage ; the outermost of these 

 laminae is the smallest, and the size of the plates increases pro- 

 gressively from without inwards, 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 



1 See his elaborate memoirs, ' On the Microscopic Structure of the Scales and 

 Dermal Teeth of some Ganoid and Placoid Fish,' in Phil. Trans. 1849 ; and ' Investi- 

 gations into the Structure and Development of the Scales and Bones of Fishes,' in 

 Phil. Trans. 1851. 



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