384 



COLLEGE ZOOLOGY 



Figure 257. Winterkill of fish due to suffocation. This is a scene in spring on the shore of a 

 northern lake. The first complete freezing over of a lake seals the water away from further 

 contact with the air, an important source of oxygen. Then when the snow on top of the ice 

 becomes deep enough to cut off the light necessary for photosynthesis in plants, an additional 

 source of oxygen is eliminated. The fish, robbed of their life-sustaining oxygen, die, as is 

 shown in this photograph. (Photo by Ouradnik. Courtesy of Institute for Fisheries Research, 

 Michigan Department of Conservation.) 



fishes the air bladder may serve in sound 

 production. It is lunghke in the lungfishes. 



Excretory system 



The kidneys lie just beneath the backbone 

 in the abdominal cavitv. They extract urea 

 and other waste products from the blood. 

 Two thin tubes, the urinary ( mesonephric ) 

 ducts or "ureters," carry the excretory matter 

 into a urinary bladder (Fig. 255), where it 

 is stored for a time and then expelled 

 through the urogenital opening, which is 

 located just posterior to the anus. 



Nervous system 



The brain of the perch (Figs. 255 and 

 392) is more highly developed than that of 

 the cyclostome or shark. The 4 chief divi- 



sions are well marked: the cerebrum, optic 

 lobes, cerebellum, and the medulla. The 

 brain gives off cranial nerves to the sense 

 organs and other parts of the anterior por- 

 tion of the body. The nerve (spinal) cord 

 lies above the centra of the vertebral column 

 and passes through the neural arches of the 

 vertebrae. Spinal nerves arise from the sides 

 of the spinal cord. 



Sense organs 



The principal sense organs are: (1) cu- 

 taneous sense organs, (2) olfactory sacs, 

 (3) ears, and (4) eyes. The integument, es- 

 pecially that of the lips, serves as an organ of 

 touch. Dermal barbels on some fishes such 

 as the catfishes also function as sensory or- 

 gans for locating food. The lateral line con- 

 tains sensory cells which serve to detect 



