152 OUTLINES OF ICHTHYOLOGY. 



the neck with gills (branchice, Latin), consisting of 

 arches of bone, to which the filaments, or spongy 

 portions, of the gills are attached, and having 

 their surfaces covered by a tissue of innumerable 

 blood-vessels. 



Of these branchial or gill-arches, there are in 

 fish five, of which four support the gills, the fifth 

 being appropriated to the " dentiferous pharyn- 

 geal arch," a long arch in the throat, bearing teeth. 

 In breathing the water is taken in at the mouth, 

 and, after passing through the gills, is allowed to 

 escape behind by the gill-openings. In its passage 

 through the gills and gill filaments, the water im- 

 parts to the latter the oxygen of the air which it 

 contains, and receives carbon in return, as in the 

 lungs of an air-breathing animal. The gills do not 

 decompose the water, so as to derive oxygen from 

 it, but merely separate the oxygen from the com- 

 mon air contained in the water. Hence, if water 

 is deprived of this air, or impregnated with un- 

 wholesome gases, fish cannot exist in it. 



The gills also possess the power of imbibing 

 oxygen not only from the portion of the air which 

 is mixed with the water, but also directly from the 

 atmospheric air itself; and this process may fre- 

 quently be noticed in a vivarium, or other recep- 

 tacle for fish, where the water is foul, or not 

 changed sufficiently often, when the fish will be 

 observed to remain for long periods together with 

 their noses protruding out of the water. The 



