270 



GENERAL BIOLOGY 



are five pairs of enlarged vessels 

 called aortic arches, aortic loops, or 

 "hearts," running from dorsal trunk 

 to 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 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. These valves are just behind the open- 

 ings 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 these just 

 mentioned are most important to show how and why the blood flows as 

 it does. 



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



S/i 



Fig. 167. 



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; s.s. septa. (After Bourne 

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



