438 COLLEGE ZOOLOGY 



cavity; it is not capable of independent movement, but func- 

 tions as a tactile organ. The mouth cavity is followed by the 

 pharynx, on either side of which are four gill-slits. Food passes 

 directly to the stomach through a short oesophagus. 



Digestion is begun in the stomach by the fluids secreted by 

 its walls. The partially digested food then passes through the 

 pyloric valve into the intestine. Three short tubes, called pyloric 

 cceca, open into the intestine and increase its secreting surface. 

 The liver lies in the anterior part of the body-cavity; its secretion, 

 the bile, is stored in the gall-bladder and then passed into the 

 intestine through the bile-duct. About the intestine, which 

 curves slightly in the body-cavity, is a mass of fat. Undigested 

 substances pass out through the anus. A large red gland, the 

 spleen, is situated near the anterior end of the intestine ; it has no 

 duct. 



The Circulatory System. — The blood of the perch contains 

 oval nucleated red corpuscles and ameboid white corpuscles. The 

 hv*rt lies in a portion of the coelom, the pericardium, beneath the 

 pharynx. Circulation in the perch is similar to that in the dog- 

 fish shark (Fig. 361). Blood is carried into the thin-walled 

 auricle (au) from the veins through the sinus venosus (s.v). 

 It passes into the muscular ventricle (v) and is forced by rhyth- 

 mical contractions into the ventral aorta (v.ao) and thence by 

 afferent branchial arteries (a.br.a) into the gills. The aerated 

 blood is collected by the efferent branchial arteries (e.br.a) and 

 conveyed to the dorsal aorta (d.ao). Various parts of the body 

 are supplied by branches from the dorsal aorta. Oxygen is sup- 

 plied to the tissues by the arterial capillaries, and waste sub- 

 stances are taken up by the venous capillaries and transported 

 to the excretory organs. Veins carry the blood back to the 

 heart. Circulation is much slower in fishes than it is in the 

 higher vertebrates. 



The Respiratory System. — The perch breathes with four 

 pairs of gills supported by the first four gill-arches. Each gill 

 bears a double row of branchial filaments (Fig. 373) which are 



