FISHES. 121 



which consists of the fishes, we find interesting struc- 

 tures in the blood, the scales, and the roe. The corpus- 

 cles of the blood (PL IX. fig. 4) differ from those of the 

 Mammalia, but agree with those of birds and reptiles, 

 in being oval instead of round. The scales of fishes 

 (PL IX. figs. 22, 23) are usually rounded or oval, 

 as in most of our freshwater fishes, when they are 

 called cycloid (KVK\OS, circle) ; but sometimes they 

 are toothed at one end (fig. 23 a), forming ctenoid 

 (/tret?, a comb) scales, as in the perch. Most scales 

 exhibit a number of concentric rings, which are the 

 indications of lamince ; and many of them are lobed 

 at the margin, sometimes also having radiate furrows. 

 In the centre are often seen little rounded solid 

 bodies, having somewhat the appearance of cells, 

 which are very well seen in the scales of the perch ; 

 and in some scales these bodies are arranged in 

 concentric rows throughout the substance, as in 

 those of the eel or the cod (fig. 24). The substance 

 of which scales consist is generally cartilaginous ; in 

 some of them, however, true bony matter is present. 

 Fish-scales are contained within the substance of the 

 skin, and not merely attached to it by one end, as 

 appears to be the case in many fishes. In most of 

 our common fishes, as the roach or perch, the scales 

 project beyond the level of the skin ; but the projecting 

 portion is covered by a thin layer of the skin ; and 

 when the scales are scraped off, this layer, with its 

 elegant stellate pigment- cells, is usually found ad- 

 herent to it. In some other fishes, as the cod and 

 eel, the scales are entirely sunk below the surface ; 

 and these are commonly supposed to have no scales. 

 They may, however, be easily found by dissection, 

 or by drying a piece of the skin under pressure be- 

 tween two plates of glass, and mounting a portion 

 in balsam. 



The beautiful silvery lustre of the skin of fishes 

 depends upon the presence of innumerable very minute 



M 



