752 PROFESSOR J. STEPHENSON ON 



and when this occurs, one or several such antero-posterior contractions usually alternate 

 with one or several in a postero-anterior direction. On one occasion a long series of 

 antero-posterior contractions was observed during a period when the antiperistaltic 

 movements of the stomach were in abeyance. 



The conditions which exist in this form may be summarised as follows : — 

 Anatomically, the degree of differentiation attained by the vascular system is rather 

 greater than in the case of A. hemprichi, inasmuch as the dorsal vessel has lost the 

 strands which pass across its lumen, and a definite dorsal channel may also be discover- 

 able on the anterior part of the intestine. Physiologically, the contractions of the 

 intestinal vascular network and the antiperistalsis of the gut are essentially the same 

 phenomenon; and anteriorly where, in the oesophageal region, the " heart " has more fully 

 differentiated itself from the alimentary wall, its contractions, while they may frequently 

 be initiated by the antiperistalsis of the alimentary tract behind it, are in the absence 

 of this relation irregular in rhythm and variable in direction ; it is as if the contractile 

 tissue of even this part of the circulatory system had not yet become habituated to 

 independent action. 



Enchytr^id^. 



In the Enchytrseidae, the main portion of the circulatory system is represented by 

 a close network of vessels in the intestinal wall, the intestinal plexus or so-called 

 perienteric sinus. In the posterior part of these worms, there is no dorsal vessel ; but 

 at or near the anterior end of the intestine the intestinal plexus comes to an end, and 

 the dorsal vessel arises, continuous at its origin with the anterior end of the plexus. 

 The dorsal vessel bifurcates anteriorly, the branches being continued downwards and 

 backwards, and uniting to form the ventral vessel. There are a number of lateral 

 commissural vessels in the anterior part of the animal which join the dorsal and ventral 

 vessels. The ventral vessel is continued backwards to the posterior end of the body ; 

 it does not enter the intestinal plexus, but is a distinct vessel, lying either on the gut 

 or separated from it by numerous " windows,'' as far as the hinder end. The dorsal 

 vessel is contractile, the ventral not. 



The relations of the alimentary and vascular contractions in the Enchytrseidse may 

 be illustrated by observations on a worm belonging to this family, which I examined in 

 Lahore some time ago. Enchytrseids are not common in India, and this specimen not 

 being sexually mature it was impossible to identify it ; it probably, however, belonged 

 to the genus EnchytrsBUS. 



The dorsal vessel arose from the gut plexus in the twelfth segment, and could be 

 followed, attached to the wall of the alimentary canal, as far forwards as segment iv. The 

 antiperistaltic contractions of the intestine, which are here and throughout the Encliy- 

 trseidse the means by which the blood is moved along the channels in the intestinal wall, 

 were sluggish and of small amplitude ; on arriving at segment xii. the contractions of 

 the gut were after a momentary halt taken up by the dorsal vessel ; the contraction of 



