156 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



shell walls themselves, and the floor is formed by a membrane sup- 

 ported on, and investing in part, the strong muscles which connect 

 the abdomen with the upper anterior part of the dorsal plate. Thus 

 a space is left between the pericardium and the intestine which is oc- 

 cupied by a special blood sinus leading toward the posterior and 

 lower part of the abdomen. The posterior wall of the space is formed 

 by a chitinous partition which bounds the brood space, or its homo- 

 logae, and is connected by chitinous processes (stutzbalken) with the 

 outer skeleton. The anterior, on the other hand, is only bounded by 

 the supporting ligaments of the abdomen above described and mem- 

 branous partitions. As usually described, the heart lies suspended in 

 the cavity thus defined by slender muscular threads, more or less like 

 those of the heart of Corethra larvae and the like; and such seems to be 

 the case at first, but a more careful study shows that this is far from 

 correct. On the contrary, the chief supports of the heart are mem- 

 branes which, seen in cross-section with the attached grains or blood 

 globules, assume the appearance of exceedingly slender structureless 

 threads. The action of re-agents indicates that these supposed 

 threads are not muscles, but composed of connective tissue; Mobile by 

 changing the focus the sharpness of the line is frequently not altered, 

 but Us relative position is changed, — a simple test which often serves to 

 dispel an illusion of this sort. That there are some threads of the 

 character above mentioned is not to be doubted, as in connection 

 with the valves of the heart; but the proper support of the heart is 

 found in the membranes which invest it in part, and are reflected 

 upon the walls of the shell and, anteriorly, of the intestine. It is not 

 yrtt possible to fully describe the insertion of these tissues, as there is 

 so l.irge a number, especially about the anterior opening, where they 

 lie in all directions and at all angles, and are so transparent that only 

 their vertical sections appear as dark lines. Thus the same membrane 

 appears and disappears, only to re-appear in a different position, 

 where it might be readily taken for a distinct membrane. In general, 

 however, I hope to make no serious error in the following summary. 

 B-ifore going into detail, however, it will be necessary to consider the 

 intimate structure of the heart, as well as its general shape and posi- 

 tion. (See Plate LI.) 



The general shape is that of an irregular ov^al with the greatest 

 convexity posterior (Daphnia, etc.), or it may be strongly bifid and 

 thus somewhat Y-shaped (^Eurycercus, etc.). It is held in position in 

 the pericardial cavity by the membranes above alluded to, to which, 

 it is attached at definite points, the principal of which are two slight 

 enlargements on the lower posterior portion, which are in part op- 

 posed to each other and also to a superior posterior point of insertion. 



