THE EARTHWORM 



293 



largely of undigestible mineral matter from the soil consumed in 

 the excavation of the burrow. They are deposited on the surface 

 of the ground as the castings, to which reference has been made in 

 the account of the action of the worms upon the soil. 



Circulatory Systeyn. — The circulatory system of the earthworm 

 is more highly developed than might be expected in an animal 

 whose general organization is not elaborate (Fig. 137 C and 141). 



f"^ f'- il 



777. S- 



rm 



Fin. 141. — Transverse section of earthworm in posterior region, showing 

 blood vessels and other features not shown in Fig. 140. 



o, 8et»; h, mucous membrane of intestine; cX., ventral nerve cord; cm, commissural 

 bloodvessel; «. a., afferent intestinal blood vessel; fp, epidermis or hypodermis; ex, blood 

 vessels of nephridium; ii, lateral intestinal blood vessel; i.s., supra-intestinal (dorsal) 

 blood vessel; m.c, circular muscle layer; m.l., longitudinal muscle layer; m.a., septum; 

 n.a., circum-neural opening in septum; n.ig., interganglionic nerve; nl., lateral neural blood 

 vessel; n.s., sub-intestinal or ventral blood vessel; n.s.', sub-neural blood vessel; r.m., 

 retractor muscle of setae; s, seta; sg. nephridium; gs.e., nephridiopore or external aperture 

 of nephiridum; I.e., chloragogue tissue; ip, typhlosole. (From Howes, "Atlas of Zo- 

 otomy," copyright, 1902, by Macmillan and Co., Ltd., reprinted by permission.) 



The blood is ]:)right red from the presence of hmnoglobin, contained 

 in the plasma, and not in corpuscles as in vertebrate animals. 

 The principal blood vessels are the dorsal and ventral vessels lying 

 along the digestive tract, the sub-neural below the nerve cord, 

 and two lateral neurals. These longitudinal trunks are con- 

 nected by a complex arrangement of lateral vessels after the 

 manner shown. There is no true heart, since the larger ves- 

 sels contract by peristaltic waves, which, together with the valves 



