ENTOMOSTKACA 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 Daphnidie, as stated by Claus, there seems to be 

 but a single layer of muscular rays, but in D, schceft'eri 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. Xay, 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- 



