266 E. S. GOODRICH. 



many"as five such sets of valves ; the number of cells round 

 each constriction diminishes the farther it is from the longi- 

 tudinal vessel. It is this vessel (of the 10th segment) which 

 supplies the^blood to the ovisac, into which it passes directly 

 from the 11th segment. Both the longitudinal and these 

 lateral dorsal vessels are contractile. 



The non-contractile longitudinal ventral vessel runs straight 

 up to the 3rd segment, where it divides and joins again, forming 

 a ring. From the anterior border of this ring are given off 

 three vessels, which join the terminal dorsal branches under 

 the brain (fig. 5). The ventral system has no valves. 



The circulation of the blood (which is red) and the action of 

 the valves are as follows : The blood is propelled forward in 

 the dorsal longitudinal vessel by the contraction of its walls ; 

 it is helped in its course by the position of the valves, which 

 prevent its returning, for when the flow is forwards they 

 assume the position a' (fig. 11), leaving a clear passage; when 

 the blood tries to return they take up the position h' (fig. 11), 

 meeting in the middle and closing the way. Occasionally the 

 return flow is too strong, and the valves are forced right back 

 to the position c' (fig. 11), when they more or less completely 

 shut off the communication with the side branches. The 

 course of the blood-stream in these branches is from the longi- 

 tudinal vessels outwards to the body-wall. As already men- 

 tioned, coming off from the lateral branches there is over the 

 whole surface of the body- wall of the first 10 segments under- 

 lying the coelomic epithelium a fine network of blood-vessels, 

 which must form a very efficient respiratory system. Thus we 

 see that, in the main, the course of the blood is up the longi- 

 tudinal dorsal vessel, down the anterior lateral vessels, spreading 

 over the body-wall to be aerated, and returning to the ventral 

 vessel, in which it will flow backwards. Thence it probably 

 reaches the dorsal vessel again by means of the small branches 

 given off to the intestine, where nourishment is no doubt 

 absorbed, and so brought into the general circulation. As will 

 be seen later, excretion probably takes place from the ventral 

 vessel by means of the nephridia which cling closely to it. 



