30 EVENINGS AT THE MICROSCOPE 



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. a); 



SCALES OF PERCH. 



the middle one from the lateral line (6); 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, falling 

 on the house-top, has a tendency to flow downward, from 

 gravitation ; and, as the slope of the roof is in that direction, 



