THE CCELOMATA INVERTEBRATA 



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directed main loops. The nephrostome is situated near the mid- 

 ventral line, from 2-3 mm. from the nerve cord, and the end of the 

 tube conies back to near the middle line, and, running through the 

 body wall, reaches the exterior as the tiny hole, the nephridiopore, 

 whose position has already been pointed out. Thus a nephridium 



FIG. 57. Stereogram showing the relations of organs in the posterior 

 part of the earthworm. (Worked out by Professors McGregor and 

 Calkins, and drawn by Miss Hedge.) 



opens into the coelom of one segment, but to the outside through the 

 wall of the following one. 



The preseptal part consists of the nephrostome proper and a 

 neck-like portion. The nephrostome is a flat oval kidney-shaped 

 structure, the main part of which is composed of a single large 

 crescent-shaped cell, the central cell. Around the outer margin 

 of this is a row of slightly wedge-shaped columnar cells, the marginal 



