ENTOMOSTRACA OF MINNESOTA. 155 
and other organs. As there is no rapid circulation of blood, these 
‘‘oil drops”’ are comparatively stationary, and yet are moved slowly 
by the constant contraction of the walls of the alimentary canal, which, 
in the anterior part, or stomach, are thick and glandular, while in the 
abdomen they seem to be more fitted for respiratory function. 
The above arrangement in Cyclops is correlated with its compact 
habit and thick carapace, and forms a simple starting-point for the 
study of the circulatory system in arthropods. It seems that the walls 
of the membranous blood cavity are themselves also, in places, fur- 
nished with muscles, so that the fluid is not dependent entirely on the 
vermiform or the peristaitic motions of the intestine for its escape 
from stagnation. If this be correct, we here have an indication of the 
origin of the central organ of the circulatory system. 
Bat to return to Daphnia, the heart lies in the dorsal region over 
the intestine, upon which it may be said to ride, as it were, astride, 
though, as we shall see, it is separated from the intestine by other 
organs. In Hurycercus this is most evident, as here the heart is more 
obviously bifurcate, 
The heart and circulation in Daphnia has been described more or 
less at length by many authors, in particular Claus (76) and Gruit- 
huisen (the work of this author I have not seen), while Weismann 
(’74) describes the heart:of Leptodora, and Claus (’77) that of the Poly- 
phemide. Other authors, except G. O. Sars, who elucidates some 
points in the circulation of blood in Sida, seem to have added little or 
nothing to our knowledge of this interesting subject. 
As already often described, the heart occupies a place in a definite 
space—the pericardial chamber—the summit of which is the dorsal 
shield, which, we believe, should be distinguished from the remainder 
of the so-called cephalic shield. It is usual to describe the shell of 
Daphnia as consisting of a bivalve posterior portion or ormostegite, 
and a simple anterior cephalostegite; but it seems much more proper 
to consider that portion of the shell which covers the pericardial 
space and is the point of attachment of the powerful muscles of the 
abdomen and of the membranous walls of the pericardium as a dis- 
tinct portion of the carapace, as it often evidently appears through 
the presence of a distinct suture, or, in its absence, through the pecu- 
liar sculpture of the shell. In such case it might also be proper to 
distinguish two regions on the lateral appendages of this dorsal shield, 
an upper and a lower, separated by the more or less obvious line ex- 
tending from the union of the lateral lines of the dorsal and cephalic 
shield in nearly a straight line toward the posterior portion of the 
shell, and indicating the insertion of the muscles which move the feet 
and post-abdomen, The lateral walls of the pericardial space are the 
