ORGANS OF CIRCULATION. 



the blood for distribution to all parts of the body. The refuse 

 remaining in the alimentary canal (and which has never been a 

 part of the body proper) is finally voided through the anus as 

 castings or faeces. This process of " defaecation " must not be 

 confounded with that of excretion, which will be described later. 



Circulatory System. The food, having been absorbed, is 

 distributed throughout the body by two devices. 



1. Cwlomic Circulation. The cavity of the coslom is filled 

 with a colorless fluid (" coslomic fluid ' ') which must be regarded as a 

 kind of lymph or blood. By the contractions of the body-wall, as 

 the worm crawls about, the ccelomic fluid is driven back and forth 

 through all parts of the coelom, 

 through irregular openings in the 

 dissepiments. As the digested 

 food is absorbed from the stomach- 

 intestine a considerable part of it is 

 believed to pass into the coelomic 

 fluid, and is thus conveyed directly 

 to the organs which this fluid 

 bathes. The coelomic fluid is com- 

 posed of two constituents, viz., a 

 colorless fluid called the plasma, 

 and colorless isolated cells or cor- 

 puscles which float in the plasma, 

 and are remarkable for the fact 

 that they undergo constant though 

 slow changes of form. In fact they 

 closely resemble certain kinds of 

 Amaebce, and we should certainly 

 consider them to be such if we 

 found them occurring free in stag- 

 nant water. We know, however, 

 that they live only in the plasma, and have a common origin 

 with the other cells of the body ; hence we must regard them 

 not as individual animals, but as constituent cells of the earth- 

 worm. The coalomic fluid is in fact a kind of tissue consisting 

 of isolated colorless cells floating in a fluid intercellular substance. 

 These free floating cells are probably the scavengers (phagocyte*} 

 of the body, devouring and destroying waste matters. Some 



FIG. 25. Phagocytes, from the coe- 

 lomic fluid of the earthworm. A, 

 agglomeration of phagocytes, 

 surrounding a foreign body; B, 

 single phagocyte, with vacuoles. 

 (After Metschnikoff.) 



