THE ANATOMY. VASCULAR SYSTEM 67 



dealing respectively with Pontoscolex and Megascolex by PERRIER and BOURNE. The 

 following account is deduced from these two memoirs. In Megascolex the dorsal 

 vessel bifurcates anteriorly in the first segment of the body ; each of the two branches 

 into which it divides is the equivalent of the succeeding dorso-tegumentary vessels, 

 and like them forms a peripheral vascular network which only communicates 

 indirectly with the ventral vessel. In Pontoscolex (PERKIER) the dorsal vessel extends 

 as far forward as the brain ; there it bifurcates and joins the branches formed by 

 the bifurcation of the ventral vessel, thus forming ' a vascular collar in front of 

 the nervous collar.' In these genera, in Lumbricus, and in fact in all the higher 

 Oligochaeta the dorsal vessel gets to be very much less in calibre anteriorly and 

 communicates indirectly with the ventral longitudinal trunks ; in these Oligochaeta the 

 vascular system as a whole is, as will be pointed out, excessively complicated as 

 compared with the lower forms ; the fact therefore that in the lower forms the dorsal 

 and ventral vessels communicate directly by vascular arches similar to those which 

 occur in succeeding segments is not to be looked upon as a difference of importance, 

 but merely as a result of the more highly-developed vascular system of the former. 

 In the Tubificidae, for example, there is in the first segment of the body a vascular 

 arch connecting the dorsal and the ventral vessels directly and quite similar to the 

 following arches which put these vessels into communication in ensuing segments ; 

 but these facts will be fully gone into in a subsequent section. Posteriorly the dorsal 

 vessel appears gradually to fade away in some worms ; in others, as in Megascolex, 

 for example, it terminates abruptly just after giving off the last pair of dorso- 

 tegumentary trunks. 



A remarkable condition of the dorsal vessel was first described by myself in 

 Megascolex coeruleus ; in this worm the anterior part of the tube is partially divided 

 into two halves which reunite at the septa : a large number of earthworms are now 

 known which exhibit this peculiarity. It is, moreover, not a question of systematic 

 position ; the most diverse families show this condition ; it has been found, for 

 example, in Octochaetus multiporus, Acanthodrilus novae-zelandiae, Microckaeta rapj"', 

 Teleudrilus ragazzii, etc. Three degrees of the division of the dorsal vessel exist ; 

 in Megascolex, for instance, the vessel is only double in the anterior part of the body ; 

 in other types the dorsal vessel is double from end to end of the body ; here again 

 there is a difference ; in Octot haetus multiporus and in Acanthodrilus anvectens the 

 vessel is completely double ; there are two distinctly separate tubes running side 

 by side on the dorsal surface of the gut ; in Acanthodrilus novae-zelandiae there 

 are two such tubes in the middle region of each segment, but at the septa the two 

 tubes fuse. The interest attaching to these facts lies mainly in that they seem to 



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