THE ANATOMY OF PENTASTOMUM TERETIUSCULUM. 31 



Once the wall of the blood-vessel is pierced and a flow of 

 blood produced, the latter would be drawn up the pharynx 

 and oesophagus by the alternate contraction and expansion of 

 the walls, as described in the section dealing with the ali- 

 mentary canal. From the cesophagus it would pass into the 

 mid-gut, and its backward flow be hindered both by the 

 sphincter muscle at the base of the papilla, and by the 

 situation of the opening at the apex of a papilla. 



Under ordinary circumstances, blood, when withdrawn from 

 the vessels in which it normally flows, rapidly coagulates. Were 

 it to do so after passing into the alimentary canal of the 

 Peutastomum, quite apart from the increased difficulty of 

 digestion after coagulation, a block would be formed and the 

 further passage stopped. 



Such an animal, for example, as Hirudo, which feeds, as 

 does this Pentastomum, exclusively upon blood, is provided 

 with special structures to prevent the coagulation of the 

 blood. It seems, therefore, natural to expect that we should 

 find in Pentastomum, as in Hirudo, some structure which 

 secretes a fluid having the property of preventing coagulation 

 of the blood. This structure we may, in all probability, 

 recognise in the large head, hook, and parietal glands. 



In Hirudo, however, there is present a definite series of 

 spaces — blood-vessels or sinuses — which contain fluid into 

 which passes the nutrient material formed in the alimentary 

 canal as result of digestion of the blood sucked in. In Penta- 

 stomum no such vessels or sinuses are present. The coelom 

 is extensive, and a long mid-gut is present, lined by hypoblastic 

 cells, though no vessels are present in its walls containing 

 fluid into which can pass the elaborated food material. In- 

 stead of this, the blood which the parasite sucks in percolates, 

 in P. t ere tins culum, at any rate, through the walls of the 

 canal into the coelom and fills the latter, giving to the whole 

 body a brilliant red colour when alive.^ Either this is the case, 



1 This red colour is not due simply to the alimentary canal being seen 

 through the skin. In large females, in which the reproductive organs fill up 

 and swell out the body, the whole of the latter is brilliantly red coloured. 



