80 ALFRED GIBBS BOUENE. 



sphincter (they appear to have such a structure), and when the 

 heart contracts blood cannot flow back into either vessel. 

 Again, the muscular bulbs at the distal extremities of the 

 hearts probably act like sphincters, and ensure distension of 

 the hearts during their diastole, so that the systole has greater 

 effect ; aud after the systole their contraction prevents regurgi- 

 tation from the ventral vessel. Again, it is interesting to recall 

 here what is stated above with regard to the intestinal capillary 

 networks. Blood flows from the external network into the 

 lacunar spaces, forming the internal network, and at very low 

 pressure — a circumstance favorable, doubtless, to intestinal 

 absorption ; thence it drains away gradually into the dorsal 

 vessel ; indeed, I expect that there is some slight pumping 

 action exerted by the latter. Moreover, the anatomical 

 arrangements of these intestinal networks indicate in some 

 slight degree the probable direction of the blood-flow ; the 

 external network looks like an agency for distributing the blood 

 to the lacunae, and in so far as this is the case it bears out what 

 is stated above with regard to the direction of the flow. 



The peripheral networks deserve a special note in respect of 

 their triple connections. They are always supplied with blood 

 by two vessels in which there is some pressure, and so the 

 blood is pushed on into the third, which is always some branch 

 of an intestino-tegumentary vessel, and which thus always con- 

 veys the blood to the intestinal wall. The arrangement of such 

 vessels as those shown in fig. 4, g, passing from periphery net- 

 works in the region of the calciferous glands to the intestino- 

 tegumentary vessels, once struck me as presenting a little 

 difficulty. I mean that, as it must according to ray theory, 

 blood should be flowing towards the main trunk of the intestino- 

 tegumentary vessel in them, whilst it is flowing away from it 

 in branches which open close by them and go to the intestine. 

 But it must be remembered that the whole question is one of 

 relative pressure. In no part of the intestino-tegumentary 

 system can the pressure be very great, as it is only connected 

 with the contractile vessels by means of capillaries ; but the 

 vessels in question form part of a set of branches through 



