MetazoaLumbricus. 239 



absorbed by the columnar cells lining the intestine find 

 their way into the vessels which lie in the connective tissue 

 beneath. 



The circulatory system. The vessels which lie in the 

 wall of the intestine have extremely thin walls, often in- 

 deed only one cell thick, and are known as capillaries. 

 These capillaries, however, lead into a very complete system 

 of larger vessels, some of which run longitudinally, others 

 circularly round the intestine and in the body-wall. The 

 principal longitudinal vessels are three in number, one 

 large vessel running dorsally along the top of the alimentary 

 canal, and two ventrally placed, one immediately beneath 

 the intestine (subintestinal), the other close to the ventral 

 body-wall, and separated from the subintestinal vessel (as 

 we shall afterwards find) by the nervous system. From its 

 position it has been named the subneural vessel. From 

 all of these vessels numerous smaller branches are given off, 

 which run in a circular manner round the intestine or the 

 body wall. The arrangement of these circular vessels is 

 very complicated, and it will not be necessary for us to 

 do more than glance at the moie important branches. 

 Moreover the manner in which they are arranged differs 

 according to the part of the body examined. If we select 

 a somite near the middle, we find that from the dorsal vessel 

 a large vessel is given off on either side, which splits up into 

 numerous smaller vessels, ultimately becoming continuous 

 with the capillaries already mentioned as forming a net- 

 work in the walls of the intestine. The dorsal vessel also 

 gives off on either side a smaller branch, which, after giving 

 off capillaries on its way, opens into the lowest of the two 

 ventral vessels. The other longitudinal vessel also gives off 

 branches to the wall of the intestine and the organs in its 

 vicinity. 



In the anterior region of the body the arrangement is 

 somewhat different. In the first half dozen-somites the 

 arrangement is quite irregular, all the longitudinal vessels 



