ENTOMOSTRACA OF MINNESOTA. 153 



:amoug the Daphnidce. The compact Copepoda survive the vicissitudes 

 of coufinemeut with comparative immunity, but the first taiut in the 

 water destroys the delicate organism of Baphnia. The cause for this 

 may be found in the exposure of the most vital and delicate parts of 

 4ihe organism to the influences of the surrounding aqueous medium. 

 In particular the circulatory and respiratory systems, which here are 

 not easily to be distinguished one from the other, constitute a rela- 

 tively very large area of close contact with the water. It thus happens 

 that the central organs are influenced in a very short time by whatever 

 deleterious substances may be disseminated in the water. 



Notwithstanding this lack of centralization, the structure of these 

 animals is of a very considerable degree of complexity and, presenting 

 ■so many instructive modifications under circumstances so favorable 

 for study, has been very thoroughly investigated. The very trans- 

 parency which has made it possible to clear up many questionable 

 points in crustaceology from the lessons learned in Daphnia has ren- 

 dered the investigation of certain sets of organs extremely difficult, and 

 among these may be mentioned the circulatory system. The circula- 

 tion of the nutritive fluid and the general facts connected with the 

 heart were indeed early understood; but there remain many a detail 

 and some important relations which are as yet either imperfectly 

 known or entirely misunderstood. The following notes are off"ered as 

 a contribution to the, as yet incomplete, knowledge of the circulatory 

 apparatus. 



The observations were confined for the most part to Daphnia scluefferi 

 and Slmocephalus vetiilns, with occasional comparisons with Eari/eercus, 

 Fleuroxis, Pasithea and others. It is greatly to be desired that the 

 study might be carried to the Sididce, in which the larger size and 

 superior transparency would doubtless reward the search with several, 

 as yet doubtful details. The circulation of the nutritive fluid in the 

 Daphnidce, then, is somewhat complicated, but may be divided into a 

 superficial and a deep system. It must be remarked that this distinc- 

 tion is arbitrary and only used for its convenience. The one extends 

 over the entire inner surface of the carapace, while the latter is in close 

 relation with the vegetative organs, and extends into the branchial 

 vessels of the feet. The nutritive fluid which is normally colorless 

 and supplied with corpuscles of organized nutriment (it seems doubt- 

 ful if they should be called blood corpuscles), is confined for the most, 

 if not its entire, course within membranous walls of connective tissue 

 which, however, instead of assuming a definite form as "blood vessels," 

 for the most part conform to the contour furnished by the firmer 

 organs. 



