758 PROFESSOR J. STEPHENSON ON 



rhythm affecting either one only, or both, of two associated structures, . viz. dorsal 

 vessel and intestine. 



(2) At the hinder end of a chain of two animals a regular and strong antiperistaltic 

 action, accompanied by a violent ascending ciliary motion, began at the anus ; further 

 forwards the contraction was mainly of the dorsal vessel, the opposite side of the 

 intestine moving but little ; and in the pharyngeal region of the posterior animal the 

 contraction was solely of the dorsal vessel. Immediately consequent on the arrival of 

 the wave at the anterior end of the hinder zooid, a contraction was initiated at the 

 hinder end of the anterior zooid, l)ut this now manifested itself as a typical antiperi- 

 stalsis of the whole gut. I could not trace the continuitj^ of the dorsal vessel through 

 the region of approaching fission, and could not therefore see the actual continuity 

 of each wave ; but the time-relation described above was obvious and was observed 

 for some time. 



Nais communis Piguet, var. punjabeiisis. 



Anatomy of Circulatory System. — There is a gut-plexus ; and in the mid-ventral 

 line of the intestine there is a median channel which is occluded as the antiperistaltic 

 waves of the intestine pass along ; the current in it must therefore be postero-anterior, 

 as in the dorsal vessel. 



The dorsal vessel, covered by chloragogen cells, appears as a part of the alimentary 

 wall as far forwards as the oesophagus. It divides in the prostomium into two branches, 

 which reunite ventrally to form the ventral vessel at the level of the setal bundles of 

 the third segment. Four or five lateral vascular commissures are present in the 

 pharyngeal region. 



The ventral vessel is separate from the intestine except at the posterior end of the 

 animal. There is a series of communicating vessels, of some width, though short, which 

 pass, one in each segment, from the ventral vessel to the mid-ventral channel in the 

 intestinal wall. Posteriorly the ventral vessel unites with this ventral channel a few 

 segments in front of the anus, and ultimately is continued round the end of the 

 intestine by a branch on each side ; these meeting above form the origin of the 

 dorsal vessel. The blood is red. 



Relation of Contractions of Vascular System to those of the Alimentary Canal. — 

 The dorsal vessel, where it is included in the intestinal wall, is affected by the 

 antiperistaltic contractions of the latter, which cause a progressive postero-anterior 

 constriction of the vessel as they pass along ; the intestinal antiperistalsis and the 

 vascular contractions are thus, in many cases at least, inseparable components of the 

 same contractile wave. Certain variations, however, are sometimes observed : — 



(1) Of a fairly regular series of contractions which are taking place in an animal 

 under observation, some may affect the dorsal vesisel only, others the lumen of the gut 

 also. Or, during a period when antiperistalsis is temporarily in abeyance, the con- 

 tractions of the dorsal vessel may proceed fairly regularly, perhaps at the rate of about 



