132 



THE BIOLOGY OF AN ANIMAL. 



Besides the lateral vessels from the sub-neural and supra-neural there 

 are two other sets. These are (1) the clorso-lateral vessels, given off from 

 the dorsal vessel on each side in each somite on the anterior face of the 

 dissepiment, and running downwards and outwards; and (2) the ventro- 

 lateral vessels (c.l, Fig. 77), which occupy a corresponding position on 

 the lower side, arising from the sub-intestinal vessel. Thus there are four 

 sets of lateral vessels arising from the dorsal, sub-intestinal, supra-neural, 

 and sub-neural trunks, respectively. All run outwards, either in the dis- 

 sepiments or on the inner face of the body-wall, and break up into a most 

 complicated capillary network, which covers the entire inner face of the 

 body-wall, ramifying through its substance and through all the organs of 

 the body. The exact course of the blood in this network has not been de- 

 termined. 



Metabolic System. See p. 16-i. 



Excretory System. It is the office of the excretory system to 

 remove from the organism the waste matters resulting from the 

 breaking down of living tissue. This does not mean the passing 





FIG. 78. An entire nephriclium with its folds somewhat separated for the sake of clear- 

 ness, c.p, ciliated part; d, dissepiment; /, funnel at the inner opening; g.p, glandular 

 part; m.p, muscular part. 



away of indigestible materials through the anus, for such matters 

 have never been absorbed and built up into tissue. Excretion 

 means the removal from the body of matter which has really 

 formed a part of its substance, but has been used up and is no 

 longer alive. In our own bodies this function is performed 

 chiefly by the kidneys and the skin, the waste matters passing off in 

 urine and sweat. In the earthworm it is principally performed 

 by small organs called nephridia, of which there are two in each 

 somite, excepting the first three or four (Fig. 80). 



Each nephridium (Fig. 78) consists of a long convoluted tube, 

 attached to the hinder face of a dissepiment, and lying in the 



