158 GEOL. AND NAT, HIST. SURVEY OF MINNESOTA. 
ever, only operative so long as the whole system of membranes to 
which these belong is distended with fluid. If this blood cavity be 
punctured, the fluid flows out and the heart shrivels. It may continue 
to beat for some time, but it will be seen that the effort consists simply 
in a vigorous contraction which is followed by no perceptible enlarge- 
ment. 2. After the systole the blood of the heart is forced toward 
the head, whence it is prevented from re-entering the pericardial 
Space directly by the valves and the membrane inclosing the arterial 
blood. The pressure is therefore increased in all parts of the system 
except the pericardial chamber, where it is greatly diminished. The 
membranes supporting the heart are thus unusually tense, and the 
muscular effort having ceased, the walls of the heart are distended, 
and blood flows in in the direction of the least resistance through the 
two lateral openings or ventral valves of the heart. The contraction 
of the heart during the systole is not simultaneous in different parts, 
but begins by the contraction of the posterior part where, being nearly 
free, the motion is more marked. At the close of the systole the 
heart is irregularly contracted, the points of attachment above de- 
scribed being more distended than the remaining portions. The an- 
terior of the heart is rendered very difficult to study by the fact that 
its opening is covered by the muscles of the mandibles and obscured 
by the many supporting and vibrating membranes alluded to. 
It is, however, suspended by two folds of membrane which [ have 
been inclined at times to believe blood-vessels through appearances 
resulting from the confused currents flowing about them. The upper 
margin is also attached by a pair of cords directly to the superior 
part of the shell. The anterior opening or arterial valve is most per- 
plexing, and the following description which applies only to Daphnia 
schefferi must be subject to some doubt. It appears, however, that it 
has been in a measure misunderstood by previous writers, and namely 
by Claus, who compares it with that of Leptodora, which, if correctly 
described by Weismann, is not at all identical in form, but quite 
comparable with one of the sides or lips of the venous opening. It 
does not seem to be connected by a thread, as stated for Leptodora, 
with the aortal bulb, for in reality there is no aortal bulb; the heart 
simply is connected with the system of membranes which more or less 
inclose the system. ‘The floor of the so-called aortal space is a mem- 
brane which separates the outflowing stream from a current which 
flows toward the abdomen and passes directly under the arterial open- 
ing, so that it appears as though there was a stream entering the 
heart from before as well as at the sides; the arterial opening being 
nearer the dorsal part of the heart than is naturally expected, and the 
slight enlargements at the attachment of the supporting membrane 
