756 PROFESSOR J. STEPHENSON ON 



previously discussed (^olosomatidse and Enchytrseidse) there is no differentiated dorsal 

 vessel on the intestine ; the wliole of the intestinal plexus is contractile, — in this sense, 

 that the blood spaces, being contained within the muscular coats of the alimentary 

 canal, are emptied in a forward direction by the recurring antiperistaltic contractions 

 of the alimentary wall. In the Naididse a dorsal vessel is diff"erentiated in the intestinal 

 region ; still however in close connection with the alimentary canal, indeed forming 

 part of the alimentary wall, it is physiologically as well as anatomically one with the 

 latter. The antiperistaltic contractions still persist, and move the blood in the dorsal 

 vessel, as well as in the intestinal network ; but the dorsal vessel shows nevertheless 

 signs of a partial independence ; contraction is frequently more marked, and more 

 regular, along this dorsal strip of the canal than elsewhere. The actual phenomena 

 vary from species to species ; and a number of examples will illustrate the progressive 

 diff"erentiation or emancipation, anatomically and physiologically, of the vascular from 

 the alimentary system. 



The Genus Dero. 



In the genus Dei^o the homologue of the dorsal vessel is situated on the ventral 

 wall of the alimentary canal for the greater part of its length, and is closely united 

 with the alimentary wall till within a short distance of the anterior end of the animal. 

 A network of vessels exists in the intestinal wall, with, in one species examined by me, 

 a longitudinal vascular channel in the median dorsal line (the true dorsal vessel, as said 

 above, is situated ventrally). The ventral vessel is non-contractile, and separate from 

 the intestine except posteriorly, where it becomes joined to the alimentary wall ; it 

 gives segmental branches to the intestine. There are a number of contractile lateral 

 commissural vessels in the anterior part of the body. 



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

 Though the postero-anterior contraction of the dorsal vessel is usually merely a part 

 of the antiperistaltic contraction of the intestine, the following observations may be 

 cited as showing a commencing independence : — 



(1 ) A strong and regular antiperistalsis was observed, with a rhythm of twenty-seven 

 per minute, in the posterior part of one of these animals. Further forward, the intestinal 

 walls took little part in the contractions, which thus here mainly aff"ected the dorsal 

 vessel. In the anterior component of this chain of two the participation of the intestinal 

 wall became less and less, and ceased altogether at segment xiii. from the anterior end 

 of the animal, the only motion visible in front of this being that of the dorsal vessel. 



(2) In another specimen the dorsal vessel was contracting regularly at the rate of 

 twenty-eight per minute ; the walls of the intestine, though shaken by the passage of 

 the successive waves, took no part in the contractions. Further forwards the contractile 

 wave included the intestine also; the antiperistalsis was sufficiently violent to cause 

 food matters in the intestine to be shot upwards for some distance on the passage 

 of each wave. 



