PISCES, TRUE FISH 459 



at the front supported by the maxillary jaw above and the mandibu- 

 lar jaw below. The teeth serve to hold the prey or food in the mouth. 

 The tongue, which is supported by the hyoid hone, has a row of 

 papillae running along its midline posteriorly into the pharynx. The 

 pharynx is rather funnel-shaped and has four gill slits in each lateral 

 wall. The bones in the roof of the pharynx bear superior tooth pads 

 which are round or oval in shape. The esophagus is a straight mus- 

 cular tube near the posterior end of which enters the ductus pneu- 

 maticus from the air bladder. As is generally the case in fish, diges- 

 tion begins in the stomach which is saclike and continues directly to 

 the pylorus in the bullhead, but is cylindrical in perch, with the 

 pyloric portion extending from the side. Gastric glands in the walls 

 secrete enzjones which start the digestive process. In perch there is 

 a group of fingerlike pyloric caeca attached to the side of the pyloric 

 region. The mass of partially digested material passes through the 

 pyloric valve to the duodenum of the small intestine. The small in- 

 testine is shorter and less coiled in the bullhead than in the perch. 

 It receives the hile duct from the liver and possibly some small pan- 

 creatic ducts from small masses of pancreatic tissue held in the mes- 

 enteries. There is not a distinct pancreas in either the perch or the 

 bullhead. Digestion continues in the small intestine and most of 

 the absorption of food by the blood occurs through the walls of 

 the posterior portion of the ileum. Following the small intestine the 

 short broad large intestine leads to the anus where fecal material 

 is discharged. 



Circulatory System and Circulation 



The heart lies almost free in the pericardial space at the extreme 

 anterior end of the body cavity. It is composed of two principal 

 chambers and two accessor}^ ones. There is a single auricle with the 

 accessory sinus venosus leading into it, and the single ventricle which 

 leads into the accessory conus arteriosus. The blood enters the sac- 

 like sinus venosus from the common cardinal veins and hepatic veins. 

 It passes through a valve to the auricle, then with contraction of the 

 auricle to the muscular ventricle through the auriculoventricular 

 valve. The contraction of the ventricle forces the blood through the 

 semilunar valve into the conus arteriosus, thence to the ventral aorta 

 which branches into four pairs of afferent branchial arteries into the 

 gill arches. Here the blood passes through finely branched capillaries 



