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TEXTBOOK OF ZOOLOGY 



the specific gravity of the body of the animal in water and to serve 

 as a hydrostatic organ. In forms where the bladder is open to the 

 exterior the volume of air in it can be regulated by direct exchange 

 and allow the animal to take a definite level in the water without 

 effort. In the closed type, as in perch, the volume is regulated by 

 secretion or absorption of oxygen, as the need may be. 



Excretory Organs 



The kidneys of Ameiurus are similar to those in the perch. They 

 are located in the dorsal wall of the abdominal cavity, posterior to 

 the air bladder and just outside of the peritoneum. The head kidney 



— head kidney 

 (pronephros) 



pneumato— 

 cystic duct 



a i r bladder 



l_EFr OVARY 

 RIGHT OVARY 



KIDNEY 



(mesonephro^ 



Fig. 253.- 



URINARY 

 BLADDER 



OVIDUCT 



MESONEPHRIC 

 DUCT 



-Urogenital system of bullhead, lateral view. (Drawn by Titus Evans 

 from dissections by Rose Newman.) 



(pronephros) is a paired mass of lymphoid tissue in front of the air 

 bladder. The functional kidneys are composed of numerous urinifer- 

 ous tubules supplied with blood capillaries which extract urea, 

 creatinin, and other wastes from the blood. A slender mesonephric 

 tube leads from each kidney to the urinary bladder. The urine is 

 stored in the bladder and finally expelled through the urinogenital 

 pore just posterior to the anus. 



Skeletal System 



Since there are no scales on the bullhead, the exoskeleton consists 

 of the soft fin rays which support the fins. The ctenoid scales of a 

 fish like the perch are also exoskeletal. 



