THE TUBE-WITHIN-A-TUBE BODY PLAN 193 



heart 



dorsol blood 

 v«ssel 



ventrol blood 

 vessel 



dorsal vassal 



intastina 



vai>tral 

 vassal 



narva cord 



nophridiwm 

 nepbridium 



to nepbridium 



to body woll 

 lotar'al vassal Isasmant'ol vessel 

 Subneurol vessel from body woll 



Fig. 10-17. A schematic view of the circulatory system of the earthworm. In the lower portion, one segment has 



been greatly enlarged in order to show the course of the blood. 



to the intestine where digestion and absorp- 

 tion are carried on. Undigested material 

 passes out through the anus. Earthworms 

 deposit their "castings" on the surface of 

 the ground near tlieir burrows. When mil- 

 hons of worms continue this process for 

 centuries in the same areas, the result is 

 a constant inverting of the soil reminiscent 

 of plowing in agriculture. The castings of 

 the worms also greatly enrich the soil. 



The circulatory system (Fig. 10-17) of 

 the earthworm is similar to that of Neanthes. 

 However, there is an improvement in the 

 pumping system in the form of five pairs of 

 "hearts" which surround the esophagus and 

 connect the dorsal with the ventral blood 

 vessel. In addition to the peristaltic waves 

 that move the blood forward in the dorsal 

 blood vessel, the "hearts" send it by a rhyth- 



mic contraction of their walls to the ventral 

 blood vessel with considerable force. There 

 are five principal blood vessels in the earth- 

 worm which convey blood to all parts of 

 the body. The dorsal and ventral blood 

 vessels are the main vessels that carry blood 

 to and from the "hearts." The laterals, lo- 

 cated on each side of the nerve cord, 

 receive blood from the ventral vessel and 

 carry it to the subneural vessel via the 

 nerve cord. The blood then passes into the 

 segmental vessels which convey it up to 

 the dorsal blood vessel, picking up blood 

 from both the nephridia and the body wall 

 on the way. Short blood vessels extend from 

 the dorsal vessel and convey blood to and 

 from the intestine; blood also enters the 

 intestine from the ventral vessel. A careful 

 study of Fig. 10-17 shows the plan of the 



