28 TWELFTH ANNUAL REPORT. 



the insertion of these tissues, as there is so large a number, especi- 

 ally 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 sum- 

 mary. Before going into detail, however, it will be necessary to 

 consider the intimate structure of the heart, as well as its general 

 shape and position. 



The general shape is that of an irregular oval with the greatest 

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

 thus somewhat Y-shaped (Eicrucercus, 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 

 opposed to each other and also to a superior posterior point of 

 insertion. 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 considered as simple cells by Glaus, 

 and which in young individuals show very destinct nuclei of compar- 

 atively 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 

 Daphnidse, as stated by Glaus, there seems to be but a single layer 

 of muscular rays, but in D. schsefferi and Siuiocephalus 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 spheri- 

 cal appendage. In Leptodora Weismann has shown the heart to 

 consist primarily of a membrane of connective tissue, upon which 

 the muscular fibres or cells sit in somewhat the same position as 

 in Daphnia, except 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 



