ENTOMOSTRACA OF MINNESOTA. 157 
All three of these points are thus held in relation with the shell with 
which the attached membrane is connected on either side below and 
above. The membrane then extends part way along the heart wall 
towards the anterior and is then reflected to the shell wall. The result 
of this is that the pericardial space is an angular cavity opening in 
front. It would seem as though the membrane attaching the heart 
were identical with that lining the cavity itself. The heart proper is 
obviously composed of series of muscular elements, which are consid- 
ered as simple cells by Claus, and which in young individuals show 
very distinct nuclei of comparatively large size. These are arranged 
like the meridian lines of a globe, uniting above and below, thus 
forming the most effective apparatus possible for contracting the 
heart. In the smaller Daphnide, as stated by Claus, there seems to be 
but a single layer of muscular rays, but in D. scheefferi and Simoceph- 
alus I have repeatedly satisfied myself that some of the longitudinal 
rays sink below the others and form a series of longitudinal muscles, 
as stated by earlier writers. These are furnished with a nucleus 
which is frequently more or less external, appearing like a spherical 
appendage. In Leptodora Weismann has shown the heart to consist 
primarily of a membrane of connective tissue, upon which the muscu- 
lar fibers or cells sit in somewhat the same position as in Daphnia, ex- 
cept that there is not the same regularity in the arrangement. There 
are many considerations which would lead us to expect the same 
structure in Daphnia, though it is not yet demonstrated; and the struc- 
ture of the anterior opening seems to point in the same direction. At 
any rate there is a close connection between the muscular and con- 
nective parts of the heart. We have, then, in the heart of Daphnia a 
highly developed apparatus for closing it, but apparently none for its 
opening. This certainly is not accomplished by the few fibers which 
connect the heart with the shell, the very contractility of which is 
doubtful. Nay, more, these are insufficient even to hold it in its place 
in the cavity. Still less can we assume that the heart, from any 
inherent power, can open itself. This must be explained by the oper- 
ation of two factors which are interdependent, i. e., the elasticity of 
the supporting membranes and the unequal pressure of the blood in 
different parts of the body. 1. The membranes which support the 
heart are attached not at right angles, but, on the contrary, in a direc- 
tion more nearly parallel to the walls of the heart, and thus whatever 
elasticity they possess is greatly increased; and the diminishing of the 
size of the heart draws these membranes out of their position at the 
expense of their elasticity, which tends to restore them to their orig- 
inal position when the pressure is removed, in the same way a drum- 
head returns after a blow to its normal position. This factor is, how- 
