ON MEGASCOLEX CCERULEUS. 75 



vessels only. Perrier (9, p. 504) having observed the dorso- 

 intestinal vessels full when the dorso-tegumentary vessels were 

 empty, and vice versa, comes to the conclusion that these 

 two kinds of vessels play opposite roles, and decided on other 

 grounds that the blood flows from the dorsal vessel to the 

 intestinal capillaries, and towards the dorsal vessel in the 

 dorso-teguraentary vessels. Benham (4, p. 283), following in 

 Perrier's footsteps, brought forward the arrangement of the 

 valves at the points of junction of these various vessels with 

 the dorsal vessel in Microchseta in support of the theory ; 

 while Vejdovsky, as I have stated above, takes the opposite 

 view with regard to the direction of the blood-flow in the dorso- 

 intestinal vessels at any rate. 



I can confirm Perrier's observations that the two kinds of 

 vessels in question play opposite roles. I have made this 

 observation in several recently killed and opened worms of 

 large size, and also in small worms mounted whole in a live- 

 box ; further, in a large Megascolex recently killed and opened 

 I have emptied all these various vessels in one region by gentle 

 pressure with the finger, and then watched them refill them- 

 selves; and, moreover, I have cut a vessel of each kind and 

 watched to discover from which of the cut ends blood flows ; 

 and, lastly, I describe below an arrangement of valves slightly 

 different from that described by Benham for Microchseta, and 

 having, I believe, an exactly opposite function. 



Observations made in all these various ways have convinced 

 me that the dorso-intestinal and dorso-tegumentary vessels do 

 play opposite roles, but in the reverse way to that imagined by 

 Perrier. 



The blood enters the dorsal vessel in each posterior segment 

 through dorso-intestinal vessels, and leaves it by dorso-tegu- 

 mentary vessels. The single pair of dorso-tegumentary vessels 

 are always small as compared with the dorso-intestinal vessels, 

 and there are in many worms (e. g. Megascolex) two pairs of 

 these latter in many segments, so that doubtless more blood 

 enters the dorsal vessel than leaves it in each posterior seg- 

 ment, and the excess is passed forward to be sent out in the 



