Austratiom EarfJnrormi^. 211 



with the anterior end of the Jaferal ves-'^el, ''Ut their direct con- 

 tinuity i.s hard to make out. The lateral runs backwards, send- 

 ing a branch to the posterior septa of the segments in 6-9, and 

 passing below the alimentary canal as the sub-intestinal in 

 10-13, on the hinder septum*, of which hist segment it ends. 

 It receives branches from the calciferous glands, apparently 

 derived from the supra-intestinal in 10, 11 and 12. and others 

 from the alimentary canal, whose origin is less obvious in 13 

 and 9, giving branches to the posterior system of the segment 

 in 10, 11 and 12. 



llie reiifraJ is a single vessel, not clearly uuirked in the 

 anterior segments, giving branches to the ventral body wall 

 in each posterior to 13, and joining the dorsal as described. 

 The difficult point to explain is the slight connection between 

 the hearts or supra-intestinal vessel Avitli the dorsal, the only 

 truly functional branches arising from this in the region of the 

 hearts being those to the posterior septa of the segments, and 

 these are so small as to be best made out from sections. 



The explanation seems to be found on an examination of a 

 series of sections passing from segment 9 to about segment 15 

 or 16. The dorsal vessel is greatly expanded in the segments 

 immediately joosterior to the hearts, its walls having the typical 

 structure described by Beddard (p. 65), consisting of an epi- 

 thelial wall surrounded by circular and longitudinal muscles. 

 This vessel, evidently well fitted for pumping action, gives 

 marked vessels to the alimentary canal as described, but from 

 this organ the blood is passed to the ventral by very insigni- 

 ficant l)ranches. It must therefore pass along the alimentary 

 canal in the sinus-like vascular space surrounding it, to be 

 collected to the definite supra-intestinal vessel in 13 : and the 

 pumping force of the dorsal vessel must be strong enough to 

 pass it forward till it reaches the hearts, w^here fresh j^ropul- 

 sion will take place, since the connections of these vessels with 

 the dorsal are so small as to be negligible from a functional 

 point of view. The supra-intestinal, though not a very large 

 vessel, has, in the region of the calciferous glands, fairly muscu- 

 lar walls, which would be capable of propelling the l)lood, but 

 this theory does not explain the more than usually muscular 

 walls of the dorsal vessel in this region. 



