THE CATFISH. 169 



ming bladder, an organ by means of which the specific 

 gravity of the fish may be increased or diminished at will. 

 Find a duct connecting it with the esophagus. In a 

 few fishes its walls contain capillary blood vessels in which 

 the blood is aerated. It is homologous with the lungs of 

 higher animals. 



In front of the air bladder is a dark, reddish-brown 

 organ called the pronephros (or head kidney). Its function 

 is not well understood. 



Cut the duct of the air bladder, and lift out the diges- 

 tive organs. Three parts of the intestine not heretofore 

 noticed are easily distinguishable : (1) A wide portion 

 immediately succeeding the stomach, called, the duodenum. 

 Into this part the ducts from the liver and from the pan- 

 creas enter. (2) A narrow, much convoluted portion, not 

 well marked off from the preceding, called small intestine. 

 (3) A wider terminal portion, well marked off from the 

 latter by a hoodlike fold in the walls of the canal, and 

 called large intestine. 



Entirely remove the peritoneum, and find the special 

 excretory organs, the kidneys, lying dorsal to its posterior 

 portion. These are of a dark purplish or brownish color, 

 and are more or less united into one mass. They send 

 back a duct to communicate with a small white urinary 

 bladder beneath the vent, and to open to the exterior by 

 a separate pore a little farther back. 



A string of irregular, fatty bodies is often found ex- 

 tended lengthwise on either side of the median line, near 

 the kidneys. 



The Axis of the Body. Remove all these organs, and 

 observe the bones, now partially laid bare on the dorsal 

 side of the cavity they occupied. A chain of bones con- 

 stituting the spinal column extends along the median line 

 from the head to the posterior end of the body. The 



