The Earthworm 



271 



are five pairs of enlarged vessels 

 called aortic arches, aortic loops, or 

 "hearts," running from the dorsal 

 trunk to the ventral through the 

 seventh, eighth, ninth, tenth and 

 eleventh somites. 



These "hearts," as well as the 

 dorsal trunk, furnish the muscular 

 contraction and elongation of circu- 

 lar and longitudinal muscles which 

 force the blood through the vessels. 

 Such rhythmic contraction and ex- 

 pansion in either blood vessels or 

 intestines is known as peristalsis 



( )• 



In the frog there is a systemic 

 and pulmonary circulation. The 



earthworm, possessing no lungs, can 

 have no pulmonary circulation. 



The blood of the earthworm is 

 continuous in closed blood-vessels, 

 so it is called a closed systemic cir- 

 culation. 



But, just as there is the closed 

 circulation consisting of heart, 

 arteries, veins and capillaries in the 

 frog, as well as a lymphatic, open 

 circulation, by which the lymph passing out of the blood-vessels is able 

 to bathe every part of the body, so we speak of a coelomic circulation 

 in the earthworm, which is equivalent to the lymph-like substance out- 

 side of the blood-vessels, but within the coelomic cavity of the frog. 



The blood is collected from the intestine by two pairs of vessels 

 which enter a longitudinal typhlosolar tube. This tube is in turn con- 

 nected with the dorsal trunk by three or four short tubes in each somite. 



As there are no circular muscles in the walls of the ventral trunk, 

 this cannot contract, so the propelling of blood is caused by the dorsal 

 trunk and "hearts" as already stated. This contractile ability of the 

 dorsal trunk and "hearts," together with the fact that there are valves 

 in both of these vessels which permit blood to flow forward but not 

 backward, determines the direction of flow. The valves are just behind 

 the openings of the parietal vessels and in front of the openings of the 

 hearts. There are other valves also, in some of the other vessels, but 

 those just mentioned are the more important in showing how and why 

 the blood flows as it does. 



The blood must, therefore, flow forward toward the anterior end of 



A series of diagrams to illustrate the ar- 

 rangement of the blood-vessels and the course 

 of the circulation in Lumbricus herculeus. 

 A. Longitudinal view of the vessels in somites 

 8, 9 and 10. B. The blood-vessels as seen in 

 transverse section in the same region. C. 

 Longitudinal view of the vessels in the intesti- 

 nal region. D. Transverse section through the 

 intestinal region. sp, supra-intestinal; sb, 

 sub-intestinal, and sn, sub-neural longitudinal 

 trunks; nl, lateral neural vessels; ht, ht, con- 

 tractile vessels or "hearts;" it, intestino-tegu- 

 mentary vessels; cv, commissural vessels; af.i, 

 afferent intestinal vessels; ef.i, efferent intesti- 

 nal vessels; ty, typhlosolar vessel; i, intestine; 

 oe, oesophagus; j-.j. septa. (After Bourne 

 from a drawing by Dr. W. B. Benham.) 



