ANIMAL STUDIES 



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. 40), 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- 

 neys to be removed. 



68. Excretion. In nearly all of the 

 segmented worms there is a pair of 

 kidneys to every segment (Figs. 40, 41). 

 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 



worm kidney. 6, blood- 

 vessel ; f, funnel open- 

 ing into body cavity; 

 o, outer opening; $, 

 septum ; w, body wall. 



