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 efibrt 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 efi"ort 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 difiicult 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 I 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 Daphnki 

 schcejferi must be subject to some doubt. It appears, however, that it 

 has been in a measure misunderstood by previous writers, and namely 

 by Glaus, who compares it with that of Leptodora, which, if correctly 

 described by Weismaun, is not at all identical in form, but quite 

 comparable with one of the sides or lips of the venous opening. Ifc 

 does not seem to be conaected by a thread, as stated for Leptodoraj 

 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 



