ON MEGASOOLEX C(ERULEUS. 77 



region of the body in front of the hearts and in that behind 

 them. 



The ventral vessel has two kinds of vessels connected with 

 it — hearts and ventro-tegumentary vessels — which communi- 

 cate with peripheral capillary networks. 



The theory with regard to the flow of blood in the ventral 

 vessel universally current is that it flows backwards along its 

 whole length. I do not believe that this is the case. By far 

 the largest amount of (I believe the only) blood coming into 

 the ventral vessel comes through the hearts^ and enters^ owing 

 to the forcible contraction of the latter, at considerable pres- 

 sure. Why should it flow backwards only ? What pressure 

 can there be in the anterior portion of the ventral vessel to 

 resist any flow in that direction ? The only pressure which 

 would tend to have this eff'ect would be caused by the flow of 

 blood from the anterior branches of the dorsal vessel ; and if 

 this blood flow into the ventral vessel it is probable that there 

 is not a sufiicient quantity of it to fill also the intestino-tegu- 

 mentary vessels, and that the blood in these vessels also flows 

 forwards. There are other reasons which render this unlikely; 

 but even supposing it were the case, we have on the one hand 

 pressure caused by the flow of blood from the dorsal vessel, 

 which passes through the dorso-tegumentary branches and 

 through peripheral capillary networks, and added to this 

 pressure caused by the flow of blood which has (according to 

 the assumption) passed through intestinal capillaries, through 

 the intestino-tegumentary trunks, and finally through peri- 

 pheral networks ; while on the other hand we have pressure 

 caused by the simultaneous contraction of all the largest and 

 most powerful hearts. There can be no doubt which of these 

 two pressures would be the greater — the latter. I conclude, 

 therefore, that with regard to the ventral vessel, all the blood 

 which enters it comes from the hearts, and that all the ventro- 

 tegumentary branches — those anterior to the hearts, as well 

 as those posterior to them — are eff'erent vessels. So far as 

 the ventral vessel itself is concerned, they carry blood away 

 from it. 



