chap. v.] ' BLOOD-VASCULAR SYSTEM. 185 



bring in oxygen and food material. In the Coelen- 

 terata there are, as we know, out-growths of the 

 enteric cavity, which, in reference to their functions, 

 are spoken of as parts of the gastrovascular 



system ; along these the digested material and the 

 water taken in by the mouth pass to the different 

 cells which line them. But the only agent in the 

 propulsion of the material is the pressure due to the 

 movements of parts of the body. 



In the Turfoellaria we observe no system of 

 vessels ; the nutrient fluid either makes its way from 

 cell to cell, or passes through the clefts and passages 

 in the tissues of the body which so often indicate all 

 that can be seen of a coalom. In the dendrocoelous 

 Turbellaria and in the Trematoda the absence of a 

 system of blood-vessels is, no doubt, largely made up 

 for by the branches of the gastric cavity (see page 114), 

 which perform the same function as the gastrovascular 

 system of the Ccelenterata. In the Tapeworms the 

 only indication of a system of nutrient vessels are the 

 delicate canals that lie internally to the longitudinal 

 excretory vessels, and contain a homogeneous plasmatic 

 fluid ; these are the plasmatic canals of Sommer. 

 From the Rotatoria, on account of the small size of 

 their bodies, a system of blood-vessels is wanting. 



The origin of the closed system of vessels is 

 involved in great obscurity, but it is, at any rate, to 

 be partly ascribed to the increase in size of the 

 organism ; this increase demands the possession of a 

 means of providing for the course of the circulating 

 medium, and affords us another example of that division 

 of labour which we constantly note as we ascend the 

 scale of organisation. This explanation does not, at 

 first sight, appear to apply to all of the Jtf emertiiiea, 

 for in the smallest members of that group we find a 

 comparatively elaborate system of not only closed but 

 also contractile vessels ; further investigation reveals, 



