268 ELEMENTARY ZOOLOGY 



to the sinus venosus through a very short thin-walled 

 vessel, the hepatic sinus. 



The kidneys, more or less united in one mass, lie in the 

 posterior part of the body-cavity along the dorsal wall. 

 Note running from each side of the kidney a ureter which 

 unites with its fellow and opens into a small urinary 

 bladder which discharges through a small opening im- 

 mediately back of the anus. 



The reproductive organs lie below the swim-bladder 

 near the posterior end of the body-cavity. If the fish are 

 caught in the spring, the greater part of the body-cavity 

 of the female is found to be filled with small eggs. When 

 mature, these eggs are deposited by the mother fish in the 

 gravel of the stream-bed where they are fertilized by the 

 sperm-cells poured over them by the male and left float- 

 ing in the water. 



The nervous system of fishes is best studied in a speci- 

 men treated with nitric acid. Carefully remove the roof 

 of the skull, thereby exposing the brain. Most anteriorly 

 make out, as in the toad, the paired olfactory lobes. 

 These are attached by long stalks to the cerebrum or 

 forebrain, which is followed by two large hollow lobes, 

 the midbrain or optic lobes. Behind the midbrain is the 

 cerebellum. Following the cerebellum is the elongate 

 medulla oblongata, which tapers backward into the spinal 

 cord. How far backward does the spinal cord extend ? 

 On each side of the brain-case about opposite the cerebel- 

 lum are located the auditory organs, each consisting of 

 three semicircular canals which lie in different planes, and 

 of the vestibule. These parts are filled with liquid, and 

 suspended in the liquid in the vestibule are small calcareous 

 bodies called otoliths or ear-stones. Running out beneath 

 from the midbrain are the optic nerves, which cross, the 

 left one connected with the right eye, the right one with 

 the left eye. From each side of the medulla oblongata 



