HAIRS, FEATHERS, AND SCALES. 17 



observed. This is to increase the resisting power, as a 

 thin board set edgewise will bear a great weight without 

 bending or breaking, provided it can be kept from yield- 

 ing sidewise. The barbules are arranged only on the very 

 edge, the upper edge, of the beard. 



We will now examine some specimens of scales of 

 Fishes, all of which are very interesting and beautiful ob- 

 jects, under low powers of the microscope ; though higher 

 powers are requisite to resolve their structure. We will 

 use both. 



The scales of almost all the Fishes with which we are 

 familiar, fall under two kinds, which have been named 

 ctenoid (or comb-like), and cycloid (or roundish). The 



SCALES OF PERCH. 



Perch affords us good examples of the former kind. On 

 this slide are three scales from the body of this fish : the 

 one on the left side is taken from the back (a) : the 

 middle one from the lateral line (b) ; and the one on the 

 right from the belly (c). In order to understand these 

 objects, we must remember that the scales of Fishes are 

 horny or bony plates, developed in the substance of the 

 proper skin, with a layer of which they are always covered. 

 In most cases (as, for example, the Perch), the hinder end 

 of each scale projects, carrying with it the thin layer of 

 skin with which it is invested ; and thus the scales overlay 

 one another, like the tiles of a house, or like the feathers 

 of a bird, and that for a like purpose. For as the rain, 



c 



