18 EVENINGS AT THE MICKOSCOPE. 



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

 thin board set edgewise will bear a great weight with- 

 out bending or breaking, provided it can be kept from 

 yielding laterally. 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 

 objects 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 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 (fig. ») ; 

 the middle one from the lateral line (&) ; 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 al- 

 ways 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, falling on the house-top, has 

 a tendency to flow downwards, from gravitation ; and 

 as the slope of the roof is in that direction, the current 

 passing over each tile is deposited from its bottom edge 

 on the middle of the next one, whence it still flows down 

 to the free edge of this one, and so in succession. So 

 the motion of the bird through the air, and of the fisb. 



