ENTOMOSTRACA OF MINNESOTA. 153 
among the Daphnide. The compact Copepoda survive the vicissitudes 
of confinement with comparative immunity, but the first taint in the 
water destroys the delicate organism of Daphnia. The cause for this 
may be found in the exposure of the most vital and delicate parts of 
the 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 oforgans 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 offered as 
a contribution to the, as yet incomplete, knowledge of tke circulatory 
-apparatus. 
The observations were confined for the most part to Daphnia scheffert 
and Simocephalus vetulus, with occasional comparisons with Hurycercus, 
Pleuroxis, Pasithea and others. It is greatly to be desired that the 
study might be carried to the Sididw, 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 
Daphnide, then, is somewhat complicated, but may be divided into a 
superficial and a deep system. It must be remarked that this distine- 
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. 
