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 peristaltic 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. 



But to return to DapJmia, 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 Farijcercus 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 Glaus ('76) and Gruit- 

 huisen (the work of this author I have not seen), while Weismann 

 C74) describes the heart oi Leptodom, and Glaus ('77) that of the Poly- 

 phemidce. Other authors, except G. O. Sars, who elucidates some 

 points in the circulation of blood in Slda, 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 uuion 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 



