THE PERCH 37 



its ventral side forms a wall across the body cavity which is tough 

 and strong, its lateral sides becoming thinner dorsally. 



Place the animal in a pan of water and examine the organs, 

 without, however, disturbing them. Note the glistening perito- 

 neum, the membrane which lines the abdominal cavity ; it passes 

 along the ventral side of the air bladder. The mesenteries, the 

 membranes which support the intestine and the other organs in 

 the abdominal cavity, are folds of the peritoneum. 



The body cavity is made up of two divisions, the larger and 

 posterior of which is the abdominal cavity, the anterior and very 

 much smaller one being the pericardial cavity. The former is lined 

 by the peritoneum and contains most of the viscera ; the latter is 

 lined by the pericardium and contains the heart. These two cav- 

 ities are separated from each other by the false diaphragm, which 

 is composed of the posterior wall of the pericardium and the 

 anterior wall of the peritoneum; it is not homologous to the 

 diaphragm of mammals. 



In the abdominal cavity the largest organ is the air bladder, 

 which extends the entire length of the cavity and occupies the 

 dorsal half of its space. In the perch it is a simple sac which is not 

 connected with the pharynx by a duct as is the case in the trout 

 and many other fishes. On the inner surface of its ventral wall 

 will be noticed a pair of red patches composed of a network of 

 capillaries. The air bladder is a hydrostatical apparatus by means 

 of which the fish can maintain its position in the water at different 

 depths without conscious effort. 



At the anterior end of the abdominal cavity and just back of 

 the false diaphragm is the large red liver ; at the posterior end, 

 running back to the anus, is the intestine, which is usually inclosed 

 in fat. The anterior portion of the intestine forms a coil lying a 

 little to the right of the median plane, within which, often em- 

 bedded in fat, lies the spleen. On the animal's left, more or less 

 covered by the hinder border of the liver, is the stomach, a large 

 cylindrical body; alongside of it are several elongated pyloric 

 appendages. 



The gonads, or genital glands, which consist of the paired testes 

 in the male and the single ovary in the female, vary much in size 



