322 THE AMERICAN NATURALIST. [VoL. XXXVI. 
4. The Parietals. — Clamping these vessels has given doubt- 
ful results. The first experiments were made by pinching 
them at about the middle of their length in a number of 
worms. The blood seemed to collect between the dorsal ves- 
sel and the forceps. More recently no results whatever have 
been obtained from the parietal vessels by this method. This 
is perhaps due to the fact that branches from the body wall 
enter the parietals both above and below the forceps. In each 
of eight worms a dozen or more parietal vessels were cut, and 
in every case in which bleeding occurred the blood flowed from 
the lower end, that is, toward the dorsal vessel. 
Owing to the numerous connections of the parietal vessels, 
it is necessary to use indirect methods to determine the course 
of blood flow in them. In several sets of experiments the 
attempt was made to determine the source of supply to the 
parietals and their place of emptying, by clamping or cutting 
other vessels. All three longitudinal trunks, dorsal, ventral, 
and neural, were simultaneously clamped, with the following 
results in each case. The dorsal vessel was filled on both sides 
of the clamp. Although the pulsations were sometimes seri- 
ously interfered with, its normal fullness was maintained. 
This is to be explained, as will appear from what follows, by 
the fact that in front of the clamps a complete circulation is 
kept up and the fullness of the dorsal maintained by the dorso- 
intestinals and parietals bot% emptying into it. On the theory 
that the parietals and dorso-intestinals carry blood in opposite 
directions, it must be expected that the dorsal would be empty 
in front of the clamp in this experiment. If only one set of 
circular vessels empty into the dorsal, its own’ pulsations 
together with the other circular vessels must certainly drain 
it of blood in front of a clamp. Behind the clamp it is to be 
expected that the dorsal should be full; but the normal fullness 
is not greatly exceeded, because the clamping of the ventral 
and neurals cuts off the supply of blood to the posterior end 
of the worm. It is also possible that the course of flow is 
reversed in the parietals or dorso-intestinals or both, just 
behind a clamp on‘the dorsal, because of the excessive pres- 
sure developed by the pulsations of the latter vessel. In this 
