CLASS OSTEICHTHYES. BONY FISHES 



383 



Circulatory system 



T/ie blood of the perch contains oval 

 nucleated red corpuscles and amoeboid 

 white corpuscles. The heart lies in a portion 

 of the coelom, the pericardial cavity, be- 

 neath the pharynx. Circulation in the perch, 

 which is similar to that in the dogfish shark, 

 is shown through the heart and gills in Fig. 

 256. Circulation is much slower in fishes 

 than it is in the higher vertebrates. 



Respiratory system 



The perch breathes with 4 pairs of gills 

 supported by the first 4 gill arches. Each gill 

 bears a double row of gill filaments, which 

 are abundantly supplied with capillaries. 

 The afferent branchial artery (Fig. 256) 

 brings the blood from the heart to the gill 

 filaments; here an exchange of gases takes 

 place. The carbon dioxide, with which the 

 blood is loaded, passes out the gill, and a 



Afferent branchial artery-. Hepatic vein 



Gill capillaries 



Efferent 



branchial 



artery 



-Hepatic portal vein 

 -Intestinal capillaries 



Head capillaries 

 Ventral aorta 

 Anterior cardinal vein 



Sinus venosus 

 •-Atrium 

 -Ventricle 

 ■Bulbus arteriosus 



lena\ portal vein 

 Kidney capillaries 

 Posterior cardinal vein 



Figure 256. Diagram of the main blood vessels of a fish (yellow perch) as seen in lateral view. 

 The unstippled parts represent oxygenated blood and the stippled, nonoxygenated blood. 



supply of oxygen is taken in from the con- 

 tinuous 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 and is carried 

 about the body. The gills are protected from 

 external injury by the gill covering or 

 operculum (Fig. 253), and from solid parti- 

 cles which enter the mouth by the gill 

 rakers. 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 it is 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, but not in perch, it may also aid in 

 respiration. The gas contained in it is a 

 mixture of oxygen, nitrogen, and carbon 

 dioxide, and is derived from the blood ves- 

 sels 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 mus- 

 cular effort. The amount of gas within the 

 air bladder depends upon the pressure of the 

 surrounding water; and, in some way, it is 

 regulated by the fish according to depth. If 

 a fish is suddenly brought to the surface 

 from a great depth, the air bladder which 

 was under considerable pressure is suddenly 

 relieved, and therefore expands, often forc- 

 ing the stomach out of the mouth. In some 



