178 AN INTRODUCTION TO ZOOLOGY 



of branches which pass out on to the sides of the oesophagus, and then 

 run forwards into the anterior plexus as the lateral cesophageal 

 vessels, and each of these also sends a short branch backwards to 

 flow into the first of the commissural vessels. 



The distribution of the branches from these trunks is best con- 

 sidered in relation to the three main capillary circulations that are 

 formed, although the exact details of both the vessels and the 

 direction of the flow of the blood in them is by no means satisfactorily 

 determined, (i) The intestinal circulation, which is, of course, 

 mainly concerned with digestion, lies in the intestinal wall. The 

 supra-intestinal trunk gives off two well-marked branches, the 

 afferent intestinal vessels, in each somite which run down on the 

 outside of the gut wall. In it they form a rich capillary network, 

 from which the blood is collected up by two efferent intestinal vessels 

 in each segment. These convey the food-laden blood along the 

 inner gut wall into the typhlosolar vessel, and this in turn is, as we 

 have seen, closely connected with the dorsal vessel itself. (2 and 3) 

 The next two systems, the dermaj^or body wall circulation and the 

 nephridial circulation, are closely~connectecT together. From the 

 sub-intestinal trunk in every segment arises a pair of vessels which 

 pass laterally for a short distance and then divide into two. One 

 branch, the afferent dermal, goes to the skin, breaking up in the sub- 

 epidermal layers, and the other, the afferent nephridial, goes to the 

 nephridium or excretory organ, and both form a very close capillary 

 network. The skin serves as a respiratory organ, neither gills nor 

 lungs being present, and the oxygenated blood is gathered up and 

 returned to the commissural vessel by a series of small dermal 

 efferents. In the nephridium the waste nitrogenous matter, and 

 probably also excess of water, is removed from the blood, and thus 

 purified it is collected by one fairly large trunk, the efferent nephridial 

 vessel, and this also flows into the commissural vessel. 



As has just been indicated, the nitrogenous excretion in 

 Lumbricus is carried out by the nephridia or segmental organs. 

 These consist of a series of complex convoluted tubes arranged 

 metamerically, one pair in each segment of the body save the first 

 three and the last. They lie in the ventro-lateral portions of the 

 coelom and are closely related to the septa. A small part of each, 

 consisting of a funnel-like opening, the nephrostome, projects into a 

 somite from the front of its hinder septum, and hence is known as 

 the preseptal portion. This communicates through the septum 

 with the much larger postseptal portion, which is a long convoluted 

 tube loosely bound together by connective tissue into three laterally 



