THE HIGHER ORGANISMS 131 



there hangs down a hollow open sac called the manubrium. 

 This is in reality its stomach, but it opens into several 

 canals that radiate from the centre and communicate 

 with another canal that courses along the margin of the 

 disc. This slightly more complex arrangement is known 

 as the water vascular system. It begins in the stomach 

 and from it carries the contents water with products 

 of digestion through the tubes and back again, thus 

 affording cells remote from the actual organ of digestion 

 an opportunity to effect an exchange of useful for useless 

 matter. 



As we ascend to the true jelly fishes we find the same 



* 



Fia. 46. Diagrammatic sagittal section of Microstomum, showing a chain of 

 four zooids produced by fission, b, Brain of the original zooid (the exponents 

 indicating corresponding structures of the more recently formed zooids); c, 

 ciliated pit; d, dissepiments indicating different stages in the separation of the 

 zooids; e, eyespot; ent, entoderm: 0,~gut; gl, glandular cells about the mouth; 

 m, mouth of the original worm. (Galloway.) 



arrangement, the only difference being in the number of 

 radiating tubes and an increasing complexity of anasto- 

 mosing branches by which the .circulating fluids are 

 permitted to come in contact with a greater number of 

 cells. The propulsive force is found solely in the mus- 

 cular movements made by the swimming animal as it 

 opens and closes its transparent umbrella. 



Among the unsegmented worms the device for dis- 

 tributing nourishment does not differ fundamentally 

 from what has already been described. It consists of a 

 water vascular system comprising two main lateral 

 tubes with many branches extending to the periphery 



