240 Elementary Biology. 



breaking up into a close network of capillaries, spreading 

 over the pharynx, body-wall, and buccal cavity. In the 

 succeeding six somites the lateral hoops, instead of being 

 delicate tubes and opening into the sub neural vessel, are 

 large, swollen, tapering sacs which unite the dorsal to the 

 sub-intestinal vessel. These vessels are known as { hearts,' 

 although, as we shall presently see, they are functionally 

 not quite equivalent to the heart of higher animals. 



The vessels contain a red coloured fluid, with a few 

 corpuscles suspended in it. Although some uncertainty 

 still exists as to the real nature of this haemal fluid, there 

 can be little doubt but that it is in most respects compar- 

 able to the blood of the higher animals. Some biologists 

 are inclined however to consider another fluid found in the 

 spaces between the intestine and the body-wall, and known 

 as the ccelomic fluid, as equivalent to true blood. The 

 coelomic fluid is colourless and contains amoeboid cor- 

 puscles, and is distinctly albuminous in its chemical com- 

 position. Probably both the haemal and the coelomic 

 fluids together perform the functions ascribed to true 

 blood. 



Circulation. There seems to be no very definite 

 mechanism for distributing the food matters absorbed from 

 the intestine through the body. It is true that the swollen 

 circular vessels in the anterior part of the body exhibit 

 tolerably definite and rhythmic contractions from above 

 downwards, but the impetus so given to the haemal fluid 

 cannot be sufficient to send it through the complicated 

 series of vessels described above. If the ccelomic fluid 

 is to be considered as the true medium employed for 

 the distribution of nourishment, then no mechanism 

 exists at all for circulation beyond the continuous wrig- 

 gling movements of the body, movements which, con- 

 sidering the spongy character of the tissues of the earth- 

 worm, may serve very effectually in lieu of the force-pump 

 commonly called a heart. 



