Mesopodrilus asymmetricus. 469 



alongside the ventral vessel into IX ; that there are no other 

 nephridia until we reach XII ; and that in this somite and in 

 each of the following ones there is but one nephridium, the 

 order of occurrence being that common in Premnodrilus 

 palustris, in which, as before stated, there is on each side of 

 the body, alternately, a single nephridium in each of several 

 successive somites. The asymmetry found to exist in these 

 two species suggests that possibly the views ordinarily held 

 as to the universality of the paired arrangement of the 

 nephridia in the Lumbriculidce may be due to the lack of a 

 careful examination of their distribution. On the other hand, 

 in Thinodrilus inconstans, in a species of Sutroa from Yellow- 

 stone Park, and in Eclipidrilus frigidus, — the only other 

 species of Lumbriculidce which have been accessible to the 

 author for study, — the nephridia are paired. 



Our knowledge of the circulatory system is necessarily 

 incomplete, since the material for the study of this species is 

 limited to serial sections of parts of two specimens ; but it 

 has been possible to ascertain several facts concerning it. 



As in many other aquatic Oligochceta, branches of the 

 vascular system are freely distributed to the wall of the 

 intestine, taking either the form of extensive plexuses or of 

 sinuses of considerable extent. The ventral vessel is forked 

 near the septum Y/YI, and in each of somites II-V its 

 branches are connected with the dorsal vessel by one pair of 

 perigastric vessels. 1 In the anterior part of each of somites 

 YI-IX a pair of lateral vessels invested by gland cells con- 

 nects the ventral vessel with the dorsal part of the intestinal 

 plexus, while in the posterior part of each, a pair of slender 

 lateral vessels without investing gland cells and having a 

 somewhat tortuous course connects the dorsal and ventral 

 vessels. The relations of the lateral vessels in X are similar 

 to those existing in YI-IX, except that the posterior vessels 

 extend backward through several somites. 



Before describing further the course of these vessels, it 



1 The lerms perigastric and gastric are appliei us by Eisen, who calls lateral ves- 

 sels lying in the ccelomic cavity perigastric, and those closely associated with the 



wall of the alimentary tract gastric. 



