Premnodrilus palustris. 461 



XLL, Fig. 3), so that the lumen is a mere slit and the lateral 

 walls are almost in contact. 



The study of the circulatory system in this species, neces- 

 sarily confined to sections of preserved material, has heen 

 incomplete and somewhat difficult. The dorsal vessel con- 

 tains an extensive "Herzkorper" of nearly uniform diameter, 

 and similar structures are present in the lateral vessels of X 

 and of a few preceding somites. The ventral vessel forks 

 near the septum III/IV. In each of somites II-IX a single 

 pair of lateral vessels connects the dorsal vessel with the 

 ventral vessel or with its anterior branches. These vessels 

 have a somewhat tortuous course and are without chloragogue 

 cells. The lateral vessels of VIII follow the spermathecje 

 into IX and sometimes farther, while those of IX extend 

 inside the sperm-sacs through a considerable number of the 

 succeeding somites. In somite X there are two pairs of 

 lateral vessels, the anterior pair connecting the dorsal vessel 

 with the ventral part of an extensive plexus of vessels in the 

 intestinal wall, and each vessel of the posterior pair extend- 

 ing posteriorly, through several somites, between the corre- 

 sponding sperm-sac and its inclosing membrane, and con- 

 necting the dorsal and ventral vessels. In somite XI and in 

 each succeeding one there are two similar pairs of lateral 

 vessels, which pass from the dorsal vessel outward and 

 downward near the body wall, and enter the intestinal 

 plexus on the ventral side of the intestine. A short blood- 

 vessel connects the ventral vessel with that part of the 

 intestinal plexus into which the lateral vessels of the anterior 

 pair open, and a similar vessel connects the ventral vessel 

 with the corresponding region of the plexus into which open 

 the vessels of the posterior pair. The lateral vessels of XI 

 and of succeeding somites have ccecal diverticula, and those 

 of somites posterior to XI are invested with a layer of chlora- 

 gogue cells. In each of a few somites at the posterior end 

 there is but one pair of lateral vessels. 



The nephridia are quite similar in structure to those of 

 other members of this group, the funnel being situated in 

 the posterior part of one somite, and the remainder of the 



