202 PHYLUM CHORDATA 



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 cavities 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. On the inner surface of its ventral wall a 

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

 noticed. 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 in- 

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

 lying a httle to the right of the median plane, within which, often 

 embedded 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, consist of the paired testes in the 

 male and the single ovary in the female, and lie dorsal to the in- 

 testine, extending from the hinder end of the cavity forward. If 

 the female is studied during the breeding season, the ovary may 

 occupy a large portion of the abdominal cavity and conceal the 

 other organs. 



In the pericardial cavity, dorsal to the thick muscles between 

 the gills, will be seen the heart. It consists of the median ven- 

 tricle, a large, muscular organ, at the sides of which appears the 

 deep-red auricle ; at the back of (dorsal to) this organ is the sinus 



