174 A N INTRODUCTION TO ZOOLOGY 



nephridia and body wall (D). The dorsal trunk also communi- 

 cates with the intestine by means of branches. Two pairs of 

 vessels pass from it to the walls of the intestine in every segment 

 (ef. i. in C). The blood is collected from the intestine by two pairs 

 of vessels which enter a longitudinal typhlosolar tube (ty. in C). 

 The latter is connected with the dorsal trunk by three or four 

 short tubes in each somite. The ventral trunk in the intestinal 

 region gives off in every segment a pair of vessels each of which 

 divides, sending a branch to the nephridium and one to the body 

 wall (sb. in D). 



STRUCTURE OF BLOOD VESSELS. The dorsal trunk and hearts 

 determine the direction of the blood flow, since they furnish the 

 power by means of their muscular walls. The wall of the dorsal 

 trunk is composed of four layers: (i) an inner epithelium of thin 

 cells, (2) a connective tissue layer; (3) a well-developed stratum 

 of circular muscle fibers, and (4) an outer covering of chlorogogen 

 cells. The walls of the hearts are similar in structure. Pairs of 

 forwardly directed valves are situated in the dorsal trunk just 

 behind the openings of the parietal vessels. The valves do not 

 prevent the flow of the blood in an anterior direction, but the 

 dorsal trunk, when constricted, is completely closed by them, 

 making the backward flow impossible. Pairs of valves are also 

 present in the dorsal trunk just in front of the openings of the 

 hearts. Other valves occur in the vessels directly connected with 

 the dorsal trunk. 



The ventral trunk does not possess a layer of circular muscle 

 fibers, and, therefore, has not the power to contract. It is 

 strengthened by a thick layer of fibrous connective tissue. The 

 sub-neural and lateral-neural trunks, as well as all the smaller 

 branches, have simply an inner epithelium and an outer connec- 

 tive tissue layer (146). 



CIRCULATION. The flow of blood in the blood vessels is not 

 segmental, but systemic. Blood is forced forward by wave-like 

 contractions of the dorsal trunk, beginning at the posterior end 

 and traveling quickly anteriorly. These contractions are said 



