STATE GEOLOGIST. 29 



demonstrated; and the structure of the anterior opening seems to 

 point in the same direction. At any rate there is a close connec- 

 tion between the muscular and connbctive 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 insufficiei>t 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 operation 

 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 direction 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 original position vfhen the pressure is 

 removed, in the same way a drum-head returns after a blow to its 

 normal position. This factor is, however, only operative so long 

 as the whole system of membranes to which these belong is dis- 

 tended 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 tlje 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 enclosing 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 un- 

 usually 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, beina: nearly free, the motion is more 

 marked. At the close of the systole the heart is irregularly con- 

 tracted, the points of attachment above described being more dis- 

 tended than the remaining portions. The anterior of the heart is 

 rendered very difficult to study by the fact that its opening is 



