SCALES OF FISHES. 591 



and in the teeth of the Edentata, as well as of many Reptiles 

 and Fishes, it forms a thick continuous envelope over the whole 

 surface, until worn away at the crown. 



408. Dermal Skeleton. The skin of Fishes, of most Reptiles, 

 and of a few Mammals, is strengthened hy plates of a horny, 

 cartilaginous, bony, or even enamel-like texture, which are some- 

 times fitted together at their edges, so as to form a continuous box- 

 like envelope, whilst more commonly they are so arranged as 

 partially to overlie one another, like the tiles on a roof; and it 

 is in this latter case that they are usually known as scales. Al- 

 though we are accustomed to associate in our minds the u scales" 

 of Fishes with those of Reptiles, yet they are essentially different 

 structures ; the former being developed in the substance of the 

 true skin, with a layer of which, in addition to the epidermis, 

 they are always covered; and bearing a resemblance to cartilage 

 and bone in their texture and composition ; whilst the latter are 

 formed upon the surface of the true skin, and are to be con- 

 sidered as analogous to nails, hoofs, &c., and other " epidermic 

 appendages." In nearly all the existing Fishes, the scales are 

 flexible, being but little consolidated 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, how- 

 ever, 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 por- 

 tion 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. 307), when dried with its 

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

 powers of the Microscope (Fig. 306). Care must be taken, how- 

 ever, that the light is made to glance upon it in the most ad- 

 vantageous manner ; since the brilliancy 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 micro- 

 scopically, is the presence of a layer of isolated spheroidal trans- 

 parent bodies, imbedded in a plate of like transparency ; these, 

 from the researches of Prof. Williamson upon other scales, ap- 

 pear not to be cells (as they might readily be supposed 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 



