TUBIFEX. 319 



found in earthworms. The coelom shows no division 

 into different cavities, other than that due to segmenta- 

 tion, with the exception of the egg sac and sperm sac, 

 which are simply portions of the coelom bounded by 

 special walls and arising as simple outgrowths of the 

 coelom of certain segments. These organs will be 

 described in detail with the rest of the reproductive 

 system. 



The coelom is lined throughout by the delicate 

 peritoneal epithelium which is also reflected over the 

 other organs in the body cavity. The shape and size of 

 the cells forming this epithelium vary considerably in 

 different parts of the body, but for the present these 

 modifications in its structure will only be mentioned 

 briefly. The parietal and visceral layers are those which 

 form the innermost layer of the body wall and the outer- 

 most layer of the intestinal wall respectively. The cells 

 of the parietal layer have already been described. The 

 visceral layer is formed chiefly of much enlarged pear- 

 shaped cells — the " chloragogen cells " of Claparede. 

 This applies to the visceral layer around the intestine. 

 In the region of the buccal cavity, pharynx and 

 oesophagus, the cells of this layer very nearly resemble 

 those of the parietal layer. In certain regions of the 

 nephridial tubes the peritoneum is composed of very 

 large, vesicular cells which are very easily detached from 

 the tube and from each other. In other parts of the 

 tube, chiefly those nearer the nephrostome and 

 nephridiopore, the ordinary, flattened epithelial cells are 

 present. 



The coelom is always filled with a colourless coelomic 

 or perivisceral fluid, by which all the organs of the body 

 are constantly bathed. Suspended in the fluid are a 

 number of colourless corpuscles which may be described 

 V 



