52 AMERICAN ANGLER'S BOOK. 



as forward and upward. The iris is capable of no contraction 

 or expansion, and in order to avoid an objectionable degree 

 of ligbt, it seeks greater depths, or the shady banks of the 

 stream. As the fish may be said to have no neck, its head 

 being set immovably on its shoulders and spine, it is neces- 

 sary to change the position of its whole body, in order to 

 obtain much variety in the direction of its vision. 



The nostrils are situated between the eyes and the snout 

 they are double, and not constructed in such manner as to 

 allow the water to pass through them in breathing, that func- 

 tion being performed entirely by the gills. Notwithstanding 

 this, smell appears to be the most acute of all the senses in 

 fish, and one which contributes much to procuring their food. 



The gill-covers, in the generality of fish, are divided into 

 four parts : the preopercle, the opercle, or gill-cover proper, 

 the subopercle, and the interopercle. The opercles are in- 

 tended as a protection to the delicate organization of the 

 gills and branchiostegous rays, and open and close as the 

 water passes through them. 



That brilliant substance which imparts a metallic lustre of 

 so many hues to fish, is secreted in the dermis or skin, 

 beneath the scales ; the scales themselves are transparent, and 

 are formed of a horny substance, though, in some families, 

 the outer covering is of a bony substance, and frequently 

 covered with an enamel. The " lateral line," is a series of 

 perforated scales, which extend in most fish from the gill- 

 cover to the root of the tail. 



The gills consist of series of leaflets, suspended to certain 

 arches, termed " Os hyodes ;" each leaflet is covered with a 

 tissue of innumerable blood-vessels. The water which enters 

 the mouth escapes through the gills posteriorly, and the air 

 contained in the water acts on the blood, which is constantly 

 impelled through the gills from the heart. The venous blood, 



