586 The Animal Kingdom 



As mentioned previously the heart has three chambers. Venous 

 blood enters the sinus venosus, passes into the auricle, ventricle, and 

 out through the conns arteriosus, into the short ventral aorta. From 

 the ventral aorta, four pairs of afferent branchial arteries carry the blood 

 to the gills where gaseous exchange takes place. From the gills, 

 efferent branchial arteries join to form the dorsal aorta which distributes 

 blood throughout the body. The principal veins returning blood to the 

 sinus venosus are the anterior and posterior cardinal veins which join 

 to form the common cardinal. There is both a hepatic and renal portal 

 system. 



The four pairs of gills supported by the gill arches constitute the' 

 respiratory system of the fish. Numerous capillaries are present on the 

 double rows of branchial filaments. Oxygen-laden water enters through 

 the mouth, passes through the pharynx, out through the gill slits, past 

 the gills, and exits from the opening of the operculum. 



A large air bladder is present in the dorsal part of the coelom. It 

 is filled with a gaseous mixture of nitrogen, oxygen, and carbon dioxide 

 which is obtained from capillaries in the wall of the bladder. This 

 organ acts as a hydrostatic organ, regulating the specific gravity of the 

 fish. By regulating the amount of air in the bladder, the fish is able to 

 maintain a position in water without muscular activity. 



The long niesonephric kidneys are located dorsal to the body cavity. 

 Two niesonephric ducts remove the urinary wastes to the urinary bladder 

 where it is stored until voided through the urogenital opening. This 

 opening is just posterior to the anus. 



The yellow perch has a well-developed brain (Fig. 59) with the 

 anterior portion being smaller than that of the shark and the optic lobes 

 and cerebellum somewhat larger. There are ten pairs of cranial nerves. 

 The spinal cord connects to the medulla and passes through the neural 

 arches of the vertebrae. The spinal nerves are given off along its course. 



The sense organs include the olfactory sac, the eyes, the ears, each 

 with three semicircidar canals, and the lateral line system. 



The sexes are separate. In the male, the paired testes empty the 

 sperm into the vasa deferentia which open through the urogenital aper- 

 ture. Fertilization is external with the male spreading the milt over 

 the eggs as they emerge from the female. The thousands of eggs are 

 laid in long ribbons. These eggs are laid near shore in the spring and 

 hatch in a few weeks into the young fry which resemble the adults. 



