ON THE ANATOMY OF HTSTKIOBDELLA HOMARI. 303 



A. The First Nepliridium (figs. 1, 1, 9, 37, and 42). 



The first iiephridium arises in the segment immediately 

 behind the head, and opens on the exterior in the second 

 segment. The head of the organ 1 have never been able to 

 see plainly in the living condition, as it is hidden by the 

 muscle-bands. These are always undergoing contraction 

 •during life ; the lumen of the canal in its anterior portion is 

 continually compressed, and thus the action of its cilia 

 rendered very intermittent. The head of the organ at its 

 point of origin is very close under the epidermis; in one 

 case seeming t-o be almost under the limiting membrane of 

 the epidermis. In section the head of the organ appears as 

 shown in fig. 42, which is taken in a horizontal plane in the 

 dorsal region of the first segment. The space into which the 

 nephridial head projects is shown surrounded by a number 

 of darkly staining nuclei. The canal with its flagella is 

 shown cut in section in the body-cavity. The actual projec- 

 tion of the head into the space is not seen in this section. 



The neck segment is very clear, and were the canal pro- 

 longed into the head, as in Stratiodrilus, it could easily be 

 seen at this point passing into the head. As this can never 

 be done, it is apparent that the organ takes its origin in the 

 neck segment and is not prolonged into the head. It is also 

 •certain that it does not divide and send a branch to join one 

 from the opposite side, as in Stratiodrilus. Throughout 

 its course it is a simple, unbranched, intra-cellular tube, being 

 in the same position in both sexes. 



B. The Second Nepliridium (figs. 1, 6, 7,9, 14, and 37). 



The second nepliridium arises in the anterior portion of the 

 second sesfment, and runs back to terminate in the anterior 

 part of the third. It is much the longest, being twice the 

 length of the first. Its course is straight backwards along the 

 border of the muscle-bands. The action of its cilia is much 

 more constant than that of the others, and for this reason it 



