536 ORESSWELL SHEARER. 



tgeniatus, and the testis in the male is a much smaller 

 structure ; this allows room for the nephridial canal becoming 

 coiled, and therefore more complex in structure. In D. 

 teeniatus the muscular system is also better developed, and 

 this probably plays some part in effecting the arrangement 

 of the nephridial canals. 



In one instance the canal of the second nephridium could 

 be traced back into the next segment to about the level of 

 the head of the third nephridium, and so to the middle of 

 the segment. This relationship, however, may be somewhat 

 incorrect. It has to be remembered that the worm during 

 examination is sometimes able to crawl a little bit, in doing 

 which under the cover-glass the proper position of the dorsal 

 side of one segment above its ventral surface is displaced. 

 When the preparation is viewed vertically from above, what 

 is ventral does not correspond always to what is dorsal as 

 belonging to the same segment, and so relationships like 

 these established on living material may sometimes be in- 

 correct. In saying, therefore, that the nephridia of the first 

 and second pairs almost overlap, it must be kept in mind 

 that this is applied to living material observed under some- 

 what abnormal conditions. As the canal passes through the 

 testis it frequently passes close to a number of the large 

 vesicles in which the spermatozoa are seen actively moving; 

 these, however, are always distinctly walled off from the 

 canal, and even under great compression never rupture or 

 discharge into the nephridial cana.l. In every case the spaces 

 containing the spermatozoa are always completely shut off 

 from the body-cavity; while they are seen in almost every 

 other part of the body, they are never seen in the body- cavity 

 or in any of its connecting spaces. 



IV. The Third Nephridium. 



The third nephridium is the most characteristic of all the 

 nephridia (figs. 5, 8, 10, and 16). It is situated in the wall 

 of the body-cavity space about opposite the middle point of 



