754 PROFESSOR J. STEPHENSON ON 



and physiologically the state of things is much as in .'Eolosoma. The circulatory 

 system is anatomically on a somewhat higher level, since it possesses a series of lateral 

 commissural vessels in the anterior part of the body ; it however maintains its close 

 relation with the alimentary system, appearing in a large portion of its extent as a part 

 of the alimentary wall ; the dorsal vessel is a forward continuation of the gut-plexus. 

 Physiologically, the blood is moved, in the region of the intestine, by postero-anterior 

 contractions of the alimentary wall ; and anterior to this these contractions are con- 

 tinued forwards as the postero-anterior contractions of the dorsal vessel. Though in 

 most cases the physiological unity of the contractions of the alimentary wall and of the 

 anatomically independent dorsal vessel is unmistakable, there is in one observed case 

 [Lwnhricillus tuba) an occasional, and in another {Enchytrieus alhidus) apparently a 

 more fundamental dissociation of the two. 



Naidid^e. 



The Genus Chsetogaster. 

 Chwtogaster orientalis Stephenson. 



Anatomy of Circulator)/ Sifstem. — In describing the giU-plexus, it is convenient 

 first to distinguish the various parts of the alimentary canal. Behind a massive pharynx 

 comes the narrow oesophagus, and to this succeed two considerable dilatations, which 

 may be distinguished (46) as "crop " and "stomach" ; behind the stomach the tube is 

 continued as the intestine. 



In the wall of the crop there is on each side a series of transverse parallel vessels, 

 arising above from the dorsal vessel, and joining below a sinus-like channel in the mid- 

 ventral line. These parallel vessels appear in an optical section of the wall as clear 

 vacuole-like spaces ; they are separated from each other by intervals which are equal 

 in breadth to the diameter of the vessels themselves, and are connected with their 

 neighbours on each side by fairly frequent and distinct cross channels. Similar parallel 

 vessels occur on the stomach, connected with each other here also by cross branches ; 

 on the whole, however, the vessels are less regularly arranged and are fewer than on 

 the crop. This condition is paralleled by the optical section of the stomach wall, the 

 vacuole-like spaces being here, as compared with the crop, somewhat less conspicuous, 

 less regular, and further apart from each other. On the intestine the vacuoles represent- 

 ing the vessels of the wall are still fewer than on the stomach. 



The mid-ventral channel in the wall of the crop lias already been mentioned. 

 Anteriorly it loses itself in the network of the crop-vessels, without being connected 

 with the ventral blood-vessel. Posteriorly it is continued on to the stomach as a broad 

 channel, traceable backwards on to the intestine ; but here, though definitely present, 

 it is much less obvious. 



The dorsal vessel, contractile as far forwards as the cerebral ganglion, is closely con- 



