762 PROFESSOR J. STEPHENSON ON 



the body, and a postero-anterior contraction of the dorsal vessel in front of this. In 

 one case the contractile wave was throughout mainly a vascular contraction, i.e. affected 

 principally the dorsal vessel ; though in about the hindmost ten segments of the body 

 the lumen of the gut appeared to be slightly diminished in calibre as the wave passed 

 along, i.e. the wave was to a slight extent also an antiperistalsis of the gut. 



At the risk of insisting on the obvious, I should like to point out how the observa- 

 tions on this species illustrate the essential unity of the vascular and alimentary con- 

 tractions — how, as has been said before, they are fundamentally the same thing. 



Limnodrilus socialis Stephenson. 



I have previously (51) given a detailed account of the anatomy of the circulatory 

 system in this species. The chief points are as follows : — 



The dorsal vessel, ventral in position, as in Dero, for the greater part of its course, 

 is situated in close contact with the intestinal wall, and is surrounded by chloragogen 

 cells as far forwards as segment ix. ; it then separates from the alimentary tube and 

 becomes free in the body-cavity ; it is contractile throughout its length. A supra- 

 intestinal vessel is also present, as in many Tubificidse ; it is covered by chloragogen 

 cells, and extends forwards as far as segment v., backwards to septum 8/9, where it 

 enters a sinus-like space on the dorsal side of the intestine in the anterior part of 

 segment ix. A large channel on the right side of the intestine, also covered by chlora- 

 gogen cells, communicates anteriorly with the sinus in ix., and dies away posteriorly at 

 about segment xxi. The intestinal plexus extends throughout the length of the 

 intestine, and reaches as far forwards as iv. ; the two last-mentioned vessels (but not 

 the dorsal vessel) appear in sections as special channels of this network. 



The hearts are a single pair in segment viii, ; their pulsations have no definite tinie- 

 relation to the contractions of the dorsal vessel from which they originate above ; there 

 is also a pair of long contractile loops to the genital organs. There are lateral commis- 

 sural loops throughout the body, and these in the posterior part of the body give origin 

 to branches which go to form the cutaneous plexus ; these branches divide and anasto- 

 mose freely, penetrating the muscular coats and coming to lie between the cells of the 

 surface epithelium. 



The ventral vessel in the anterior part of the body narrows rapidly behind segment 

 vii. ; it may apparently either die away on the intestine, or be continued as a narrow 

 channel to join the ventral point of union of the two hearts. The junction of the 

 hearts may be looked on as the origin of the main ventral vessel, which extends from 

 this point to the posterior end of the animal ; it is less intimately united to the intestine 

 than is the dorsal vessel, and is not covered by chloragogen cells. 



Relation of Conti-actions of Dorsal Vessel to those of the Alimentary Canal. — The 

 antiperistaltic movements observed in this species were always quite unconnected with 

 the contractions of the dorsal vessel. The latter were much more frequent, and also 



