CLASS PISCES 



439 



abundantly supplied with capillaries. The afferent branchial 

 artery (Fig. 373, K; Fig. 361, a.br.a) brings the blood from 

 the heart to the gill- filaments; here an exchange of gases takes 

 place. The carbonic acid gas with which the blood is loaded 

 passes out of the gill, and a supply of oxygen is taken in from 

 the continuous stream of water which enters the pharynx through 

 the mouth and bathes the gills on its way 

 out through the gill-slits. 



The oxygenated blood is collected into 

 the efferent branchial artery (Fig. 373, 3; 

 Fig. 361, e.br.a) and carried about the body. 

 The gills are protected from external injury by 

 the gill covering or operculum (Fig. 372, Kd) 

 and from solid particles which enter the 

 mouth by the gill-rakers (p. 437). Because 

 oxygen is taken up by the capillaries of the 

 gill- filaments, a constant supply of fresh 

 water is necessary for the life of the fish. 

 If deprived of water entirely, respiration is 

 prevented, and the fish dies of suffocation. 



The air-bladder is a comparatively large, 

 thin- walled sac lying in the dorsal part of the 

 body-cavity. It is filled with gas and is a 

 hydrostatic organ or " float " ; in certain 

 fishes it may also aid in respiration. The 

 gas contained in it is a mixture of oxygen 

 and nitrogen, and is derived from the blood-vessels in its walls. 

 The air-bladder decreases the specific gravity, making the body 

 of the fish equal in weight to the amount of water it displaces. 

 The fish, therefore, is able to maintain a stationary position 

 without muscular effort. The amount of gas within the air- 

 bladder depends upon the pressure of the surrounding water, 

 and in some way is regulated by the fish according to the depth. 

 If a fish is brought to the surface from a great depth, the air- 

 bladder, which was under considerable pressure, is suddenly 



Fig. 373. — Trans- 

 verse section through 

 a branchial arch (B), 

 with two gill fila- 

 ments. 1, afferent 

 branchial vessel; 

 2, efferent bran- 

 chial vessel. (From 

 Schmeil.) 



