sriil'IIVIJ-^f \ EUTKHIiMW 117 



In some fishes the swim bludder still ni.iiiis cuiiiicctiitii with [\\v 

 l)h;irvn\', from which it arises, hut this comicctiuti is ctitirt'ly 

 lost in the perch. It is a hydrostatic organ ('(jualizinn the weight 

 of the animal and the water it (hsplaces. In life the swim 

 hhulder is tilled with gases. By reducing the volume of gas the 

 fish is able to descend to greater depths; hy increasing the volume 

 it is able to rise easily in the water. 



Immediately dorsad of the swim bladder and just below the 

 vertebral column are two dark-ct)lored, irregular masses extending 

 cephalad as far as the (esophagus. These are the kidneys. The 

 ureters extend the whole length of the kidneys and pass along the 

 posterior wall of the abdominal cavity to the urogenital papilla. 

 Before reaching the papilla, the two ducts unite and then enlarge 

 to form a urinary bladder located immediately above the caudal 

 end of the gonads. Jvxtend the mid ventral slit caudad on one 

 side of the anus. This will facilitate the study of the caudal 

 portions of the urogenital organs. 



The Circulatory System, — The heart of bony fishes consists of 

 but a single auricle and ventricle. The ventricle is readily 

 identified as the pale, thick-walled ventral chamber, with the 

 thin-walled, dark-colored auricle overlying its anterior portion. 

 Posterior to the auricle is the sinus venosus. From the ventricle 

 the blood passes cephalad into an enlarged vessel, the bulbits 

 aurtic, and thence into the ventral aorta which divides to form four 

 pairs of afferent branchial arteries. Each of these follows the 

 course along a gill arch to break up into the capillaries of the 

 gill filaments where an exchange of gases takes place. These 

 capillaries unite into efferent branhcial arteries which come 

 together on the middorsal line to form the principal artery of the 

 body, the dorsal aorta. This is the single type of circulation. 





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