346 



ZOOLOGY. 



bunches of feathers ; finally, in most cartilaginous fishes, as 

 in the skate and shark, they are attached by their external 

 edge to the skin, as well as by their internal to the branchial 

 arches. 



The mechanism of breathing is best observed in the living 

 fish : the mouth and gill-covers open alternately ; the water 

 entering by the mouth escapes by the openings of the gills, 

 the gills themselves having in the meantime been bathed in 

 it; whilst this happens, the oxygen is extracted from the 



Fig. 316. Hippocampus. 



water. In osseous fishes there is generally but one aperture 

 of the gills on either side : in those with fixed gills, there are 

 as many apertures as branchiae. Thus, in the shark (Fig. 317) 

 there are five pairs of openings, and in the lamprey (Fig. 334) 

 seven. Hence the nature of the respiratory apparatus may be 

 known by an inspection of the exterior. Finally, in some fishes 

 (the lamprey), the water entering by the mouth reaches the gills 

 through a kind of canal placed under the gullet, and inter- 



Fig. 317. The Shark. 



posed between the cavity of the mouth and that for the gills ; 

 the arrangement resembles in some measure the trachea of 

 the higher organized animals. 



Fishes do not consume much oxygen ; nevertheless, some 

 come to the surface from time to time to breathe air, as if 

 that absorbed by the gills was not sufficient. Some even 

 swallow it, and convert the oxygen into carbonic acid whilst 



