26 TWELFTH ANNUAL REPORT. 



drops occupy the same relative position as the blood globules of 

 other Crustacea, i. e., they lie within a very thin membrane cor- 

 responding to the vascular walls of other animals. This mem- 

 brane, in general, invests the alimentary canal, as can be very 

 readily seen in the abdomen, where it encloses a considerable space- 

 about the intestine, which is filled with fluid, investing more or 

 less completely the muscles 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 com- 

 pact 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, furnished with muscles, so that the fluid is not depend- 

 ent 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 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 Eiirycercus 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 Clans 

 (Zur Kenntniss der Daphniden und verwanter Cladoceren. 

 Zeitsch. f. Wiss. Zool. Bd. xxvii.) and Gruithuisen (the work 

 of this author I have not seen), while Weismann (Ueber Bau 

 und Lebenserscheinungen von Leptudora hyalina, 1874 ) describes 

 the heart of Leptodora, and Clans (Zur Kenntniss des Baues und 

 der Organ, der Polyphemiden), that of the Polyphemidse. Other 

 authors, except G, 0. 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 1 e 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 



