SCALES OF PISHES. 



01 



Fig. 362. 



tBmsam 



solidated by calcareous deposit ; and in some species they are so 

 thin and transparent, that, as they do not project obliquely from 

 the surface of the Skin, they can only be detected by raising the 

 superficial layer of the skin, and searching 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 encloses it, which is studded with pigment-cells ; 

 and a portion of the skin of 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. 363), when dried 

 with its scales in situ, is a very beautiful opaque object for the low 

 powers of the Microscope (Fig. 362), especially with the Binocular 

 arrangement. 

 Care must be 

 taken, however, 

 that the light is 

 made to glance 

 upon it in the 

 most advanta- 

 geous manner ; 

 since the bril- 

 liance 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 examined microscopically, is the presence of a layer of 

 isolated spheroidal transparent bodies, imbedded in a plate of like 

 transparence ; these, from the researches of Prof. Williamson upon 

 other scales, appear not to be Cells (as they might readily be sup- 

 posed to be), but to be concretions of Carbonate of Lime. When 

 the scale of the Eel is examined by Polarized light, its surface 

 exhibits a beautiful St. Andrew's cross ; and if a plate of Selenite 

 be placed behind it, and the analyzing prism be made to revolve, 

 a remarkable play of colours is presented. 



548. 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, 



Portion of Skin of Sole, viewed as an opaque object. 



