90 Laboratory Guide in Zoology 



the oesophagus to observe its union with the stomach. 

 Then follow the intestine from the stomach in its not 

 long course to the anal opening. The large, reddish 

 lobed object lying in the anterior part of the body cavity 

 is the liver. Try to make out its connection with the 

 intestine. The gall-bladder is a small, greenish body, 

 partly concealed by and closely connected with the 

 liver. 



IX. — Reproductive and Excretory Organs. In the vertebrates, 

 as in many invertebrates, the sexes are separate. So in 

 the fish we must look for testes or ovaries, never both. 

 The appearance of the female organs or ovaries varies 

 greatly with the season. In spawning time the ovaries 

 may be so distended that they cover all the other organs. 

 At other times they have the appearance of a single 

 white body lying near the intestine. A delicate tube leads 

 from this to an opening toward the posterior end of the 

 body. If the specimen is a male, the testes will be 

 found to be a pair of white bodies lying in the posterior 

 dorsal part of the abdominal cavity. Ducts may be 

 found passing to an opening toward the posterior end 

 of the body. After clearing away the intestine and 

 reproductive organs, a broad, flat, membranous sac may 

 be seen, if it has not already been punctured : this is 

 the air-bladder ; it is connected with the oesophagus. 

 Its function is to decrease the specific gravity of the fish 

 by filling it with air. Remove the air-bladder, and find 

 two long, dark, slender bodies, the kidneys. Trace their 

 ducts to the outlet near the anal opening. Near the 

 outlet a pinkish sac, the urinary bladder, will be found. 



X. — Endoskeleton and Central Nervous System. Cut the 



flesh away from the dorsal surface of the fish, expos- 

 ing the backbone as completely as possible. Notice 



