THE WORMS 57 
means of the fluid in the body cavity; but in the earthworm 
the division of labor between different parts of the body is 
more perfect, and a definite blood system now acts as a 
distributing apparatus. This consists primarily of a dorsal 
vessel lying along the dorsal surface of the alimentary canal 
(Fig. 34), from which numerous branches are given off to 
the body wall, and to the digestive system through which 
they ramify in every direction before again being collected 
into a ventral vessel lying below the digestive tract. In 
some of the anterior segments a few of the connecting 
vessels are muscular and unbranched, and during life pul- 
sate like so many hearts to force the blood over the body, 
forward in the dorsal vessel, through the “hearts” into the 
ventral vessel, thence into the dorsal by 
means of the small connecting branches. 
Some of the duties of this vascular 
system are also shared by the fluid of 
the body cavity, which is made to cir- 
culate through openings in the parti- 
tions by the contractions of the body 
wall of the animal in the act of crawl- 
ing. In this rough fashion a consider- 
able amount of nutritive material and 
oxygen are distributed to various or- 
gans, and wastes are carried to the kid- py, 35 piagram of earth- 
neys to be removed. worm kidney. 6, blood- 
58. Excretion.—In nearly all of the bdo (ae 
segmented worms there is a pair of 0, outer opening; s, 
kidneys to every segment (Figs. 34,35), ““Pttms ™ body wall 
Each consists of a coiled tube wrapped in a mass of. small 
blood-vessels, and at its inner end communicating with the 
body cavity by means of a funnel-shaped opening. In 
some unknown way the walls of the kidney extract the 
waste materials from the blood-vessels coursing over it and 
pass them into its tubular cavity. At the same time the 
cilia about the mouth of the funnel-shaped extremity are 
