STUDY OF FISHES 145 



The epidermis of fishes is characteristically slimy. There 

 are abundant glandular cells or mucus-cells, the living matter 

 of which uses itself up in making slime. 



The epidermis of fishes is transparent and comes off at touch ; 

 the dermis is the seat of the coloration. The colour is mainly 

 due (a) to various pigments contained in special cells (chromato- 

 phores) and (6) to a deposit of a waste product (guanin) in the 

 form of minute granules or discs, which are very opaque and have 

 great reflecting power. The silvery appearance and the rainbow- 

 like lustre of many fishes are due to the guanin, and it is interesting 

 to notice that this same substance is used in making artificial 

 pearls. 



Thus the skin of fishes is peculiar in having a very delicate 

 transparent slime-producing epidermis, and in having a thin, 

 practically non-muscular, dermis, with pigment and reflecting 

 grains. But the most characteristic feature is the development of 

 scales. These are so interesting and instructive that they may 

 be considered separately. 



(f) Scales. A bird is known by its feathers, and a fish by its 

 scales. Not only are the scales of fishes quite different from 

 any other scales, but some fishes at least can be quite securely 

 identified from their scales alone. A number of more or less 

 constant peculiarities make up what we call the specific char- 

 acter of a herring, or a mackerel, or a flounder, or a stickle- 

 back, on the strength of which, indeed, each of these fishes has 

 received a particular name, and in the sum-total of details one 

 of the most reliable items is what one might be inclined to think 

 the most trivial, namely, the minute structure of the scale. From 

 a single feather on the moor a skilled ornithologist can some- 

 times say that such and such a bird has been there ; from a square 

 inch of skin an expert ichthyologist can sometimes identify 

 his fish. All this means that a living creature is a distinctive 

 unity through and through. We have spoken of this rather 

 difficult subject here partly because the distinctiveness of fish- 

 scales illustrates the point particularly well, and partly because 

 we wish to suggest that a teacher need never be afraid of the 

 limitations of detailed study. If he has given some time to 

 the practical study of the scales of fishes he can make this the 



VOL. I. 10 



