54 THE BIOLOGY OF AN ANIMAL. 



suppose that they also attack invading parasites such as bacteria. 



2. Vascular Circulation. Besides the coelomic circulation 

 there is another and more complicated circulatory apparatus con- 

 sisting of branching tubes, the Hood-vessels, which form a com- 

 plicated system ramifying throughout the body. Through these 

 tubes is driven a red fluid analogous to the red blood of higher 

 animals, and like it consisting of plasma and corpuscles, the latter 

 being flattened and somewhat spindle-shaped. The red color is 

 due to a substance, haemoglobin, dissolved in the plasma and not (as 

 in higher forms) contained in the corpuscles, which are colorless. 



The earthworm is not provided with a special pumping- 

 organ or heart for the propulsion of the blood, sucli as we find 

 in higher animals. In place of this certain of the larger blood- 

 vessels (viz., the "dorsal vessel" and the "aortic arches") 

 have muscular contractile walls, which propel the blood in a con- 

 stant direction by wave-like contractions that run along the 

 vessel from one end to the other (" peristaltic " contractions, cf. 

 p. 51) at regular intervals and thus give rise to a "pulse." 

 The contractile vessels give off other non-contractile trunks 

 which divide and subdivide into tubes of extremely small calibre 

 and having very thin walls. The ultimate branches, known as 

 capillaries, permeate nearly all the organs and tissues, in which 

 they form a close network. The stream of blood after passing 

 through the capillaries is gathered into successively larger vessels 

 which after a longer or shorter course finally empty into the 

 original contractile trunks and complete the circuit. Thus the 

 vascular system is a closed system of tubes, and there is reason to 

 believe that the blood follows a perfectly definite course, though 

 this is not yet precisely determined.* 



We may now consider the arrangement of the principal 

 trunks. The largest of them, which is also the most important 

 of the contractile vessels, is : 



^ The dorsal vessel (Fig. 24, d.v.\ a long muscular tube 

 lying upon the upper side of the alimentary canal. In the liv- 

 ing worm it may be distinctly seen through the semi-transparent 



* It should be noted that in the absence of a heart it is difficult to distin 

 guish between "arteries" and "veins." We may more conveniently distin- 

 guish " afferent vessels," carrying blood towards the capillaries, and ""efferent 

 vessels," carrying blood away from them. 



