766 PROFESSOR J. STEPHENSON ON 



these animals do not live in a fluid medium ; and it has not been retained for circulatory 

 purposes, since a special propulsive mechanism, the dorsal vessel, has been diflferentiated. 

 We thus reach the end term of our present series. The anatomical development 

 and differentiation of the vascular system have attained their maximum ; and a com- 

 pletely separate dorsal vessel has taken over the function of propelling the blood, which 

 in earlier stages was effected by the antiperistaltic contractions of the alimentary wall. 



3. The Evolution of the Vascular System in the Annelida, 



The short summaries which have been appended to the accounts of the several 

 families treated of in the foregoing section have already made evident the 

 general idea which the recital of the series is intended to convey ; that is, that in the 

 lower Oligochseta the vascular system is, in its simplest form, intimately connected 

 with the alimentary tube, largely as a series of vacuole-like intercommunicating spaces 

 in the intestinal wall, in which spaces the movement of the contained fluid is determined 

 by the antiperistaltic contractions of the wall of the tube ; that in the higher stages of 

 its development it comes to show, anatomically and physiologically, a large degree of 

 independence of the alimentary tract ; and that there are a number of intermediate 

 stages which connect the simpler with the more highly differentiated condition. 



It will not, of course, be supposed that the series of forms described in the preceding 

 section is a linear series which reflects with exactitude the actual evolution of the 

 vascular system of the Annelida. It seems, nevertheless, to be capable of affording a 

 number of indications, and accordingly it may be useful to supplement the statement 

 of the general proposition enunciated above by sketching, in the light of the foregoiug 

 observations, the possible course of evolution of some of the component parts of 

 the system. 



The Intestinal Network. 



In the iEolosomatidse the intestinal network is, in bulk as well as in physiological 

 importance, tlie chief portion of the vascular system, and other vessels are few in 

 number ; the condition is not very different in the Enchytrseidse. In the higher groups 

 it loses its preponderance, owing to the greatly increased development of peripherally 

 situated trunks and capillaries. 



I propose to regard the network as the primary constituent of the vascular systenj, 

 and I believe that a recognition of its importance is a first requisite for an understand- 

 ing of the evolution of the latter. In this connection I may quote Vejdovsky (54): 

 " Das Magendarra-Gefassnetz ist iiberhaupt als Hauptbestandtheil des Gefiisssy stems 

 der Oligochseten zu betrachten " ; and in a more recent communication (55) the 

 same author speaks of the intestinal sinus as the most primitive portion of the 

 vascular system. 



The intestinal network is throughout the Oligochseta situated within the muscular 

 coat of the intestine ; it will suflice to refer to the evidence collected by Lang (29, 



