190 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



through the walls of the intestine, most of it passing into the blood 

 and directly into the coelomic fluid, where it may continue to the 

 muscular wall outside the coelom. Unusable material, mostly earth, 

 is passed off by muscular contraction through the anus, and may often 

 be seen on lawns as little piles of "castings." 



How Blood Circulates 



Since in the earthworms there is a very different arrangement than 

 in Hydra, where food is directly available to all the cells, we would 

 expect to find some means of distributing it to the tissues where it 



may be used. This is accomplished by 

 means of a closed system of blood vessels. 

 Some idea of the circulation may be derived 

 by a study of the accompanying diagrams. 

 Five large blood vessels run lengthwise 

 through the body, one dorsal vessel, close 

 to the food tube, into the walls of which 

 it sends two pairs of lateral vessels in each 

 segment ; another, the ventral vessel, runs 

 just ventral to the digestive tract and also 

 sends lateral branches into its wall. There 

 are also three others, the paired lateral 

 neural vessels and the suhneural vessel, 

 which run longitudinally, the latter directly 

 under the nerve cord, and two other smaller 

 ones lying parallel one on each side and 

 above the nerve cord. Five "hearts," so 

 called because of their frequent contrac- 

 tions, encircle the esophagus in the region 

 of the 7th to the 11th segments, connecting 

 the dorsal with the ventral vessel. Blood 

 passes into the dorsal vessel especially 

 from a long typhlosolar vessel which helps 

 drain absorbed foods from the intestinal 

 walls, flowing forward until it reaches the 

 " hearts." Its forward movement is caused 

 by slow, regular contractions of the dorsal 

 blood vessel. The blood passes posteriorly through the "hearts" 

 and then flows into the ventral blood vessel. Here it passes poste- 

 riorly, although some of it moves from the hearts toward the 



nerves 



buccal cavity 

 Esuprcicsophatfeol , 

 tiixurnssoiohigeal 

 Sub esopho^ol 



.„psai, 

 vessel 



lateral 

 vessel 



■esof>hogus 



..ventral 



VGSjel 



- - (trop 



.nsrve ccnet 

 ■with. IntM-al 

 neurai vesssis 



intastina. 



The circulatory system of the 

 earthworm. 



