PISCES, TRUE PISH 463 



where it is aerated by absorption of oxygen from the water passing 

 over the surrounding gills. This blood continues by convergence of 

 the capillaries into the efferent branchial arteries which lead dorsally 

 and join in the formation of the dorsal aorta. This is the principal 

 artery of the trunk of the body and gives subclavian branches to the 

 pectoral fins, coeliaco-mesenteric artery to the viscera, parietal ar- 

 teries to the body wall, renal arteries to the kidneys, and finally ends 

 in the caudal artery supplying the tail. The food substances and 

 oxygen are supplied to the tissues of the body by means of capillary 

 branches through them. These capillaries also collect the waste prod- 

 ucts and converge to form the veins which carry the blood back to 

 the heart. The 'posterior cardinals return from the posterior portion 

 of the trunk, the hepatic portal from the visceral organs to the liver, 

 and the hepatic from the liver to the sinus venosus, while sub- 

 clavian veins return from the pectoral fin region. The blood consists 

 of the fluid plasma, oval nucleated red. corpuscles, and amoeboid 

 white corpuscles. 



Respiratory System 



The mouth is used in forcing water over the four pairs of gills. 

 Water is drawn into the mouth by lowering the floor, while the 

 branchiostegal membranes at the margins of the opercula are closed 

 down over the gills. The opening of the mouth is guarded by fleshy 

 flaps or oral valves. When the mouth is filled with water, it is closed, 

 and these valves prevent water from escaping through this aperture. 

 The branchiostegal membranes relax and when pressure is applied to 

 the water in the mouth, it is forced out over the gills. This process 

 is repeated in rhythmic sequence in order that the blood passing 

 through the capillaries of the gills will be constantly aerated. 



The air bladder likely has some respiratory function. It is a large, 

 tough sac, taking up almost one-half of the space of the abdominal 

 cavity. The pneumatic duct extends from its midventral region to 

 the posterior part of the esophagus. There is likely some exchange 

 of air through this and the possibility of some degree of diffusion of 

 gases between this and the blood in its walls. In the perch, which 

 has no pneumatic duct, it is found that oxygen is secreted into the 

 closed swim bladder during periods of plentiful supply in the water 

 then drawn upon by the blood at times when the environmental 

 supply is scant. Another function of the air bladder is to decrease 



