OUTLINES OF ICHTHYOLOGY. 165 



Scales : Formation and Uses. — The scales of 

 fish are productions of the skin, each scale being 

 attached to it by its more forward edge, which fits 

 into a sort of pocket or " follicle ;" but the manner 

 in which the scales overlap each other is variable. 

 Those down the side, forming the lateral line, are 

 pierced in the centre with openings, to allow the 

 escape of a lubricating fluid. 



The heads of most fish also possess these 

 openings, or orifices, usually in greater numbers 

 than the other parts of the body ; and thus the 

 mucus or slime by which the scales of the fish 

 are, as it were, varnished, or waterproofed, being 

 exuded from the pores of the head is necessarily 

 carried backwards by the current — or, in still 

 water, by the progressive motion of the fish — and 

 spreads itself over the whole surface of the body. 

 The lubricating fluid is most abundant in fish with 

 small scales, such as Eels. 



Occasionally the scales are entirely covered by 

 a soft thick cuticle, so as to be detected only by 

 close examination. 



Cycloid, Ctenoid, Gafioid, and Placoid^ are the de- 

 nominations of the four different kinds offish scales. 

 The first two of these include all the most com- 

 mon forms of scales, and are marked with circular 

 or concentric lines discernible through a micro- 



' From the Greek words cidos, signifying resemblance, and 

 cycles, a circle; cteis, a comb or rake; ganos, brilliancy, and 

 plax, a flat, level surface. 



