154 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
This membrane, which is frequently folded upon itself and invests 
the body walls and the inner organs, is in some places free, and may 
be seen as a pulsating, swinging film, or, more frequently, it can only 
be detected as a swaying line (seen in optical sections), thus giving 
rise to the misapprehension that one is dealing with a thread, or as 
moving grains, in which case the film is itself invisible but its presence 
is indicated by the attached grains of protoplasm. -About the heart 
the free swaying portions of this membranous layer are so numerous 
as to render it almost impossible to distinguish the essential from the 
accidental appearances. 
This membrane must serve the most various purposes; aside from 
the mere retention and direction of the blood currents, it is often 
transformed into a branchial surface. At definite points it becomes 
the bearer of the cells which were above mentioned as grains of pro- 
toplasm. These are most numerous in young and well-fed animals, 
and in particular in gravid females, while, on the contrary, mature 
males and females after the escape of the young, are nearly devoid of 
such bodies. These are most numerous in the angles of the membrane, 
particularly about the heart, shell glands, ovaries, intestine and the 
branchial spaces in the feet. 
These cells vary in size from that of the blood corpuscles to larger 
cells with nuclei of comparatively very large size. It would be too 
much to say that such cells are developing blood corpuscles; but that 
they are reservoirs of nutriment which serve to supply the increased 
demand upon the blood in exigencies of the existence of the animal, 
cannot be doubted. It is a well-known fact that the number of blood 
corpuscles, so called, likewise varies, and apparently under the same 
conditions. It seems altogether probable that the two facts may be 
considered as supplementary, i. e., that the same process of depau- 
perating of the blood, which deprives it of its corpuscles in an earlier 
Stage, lays waste those supplies laid up in the cells referred to 
(whether by their actual separation as blood corpuscles or simply 
dissolving of the contained material is of little importance). These 
cells also. are thus parallelized with the ‘‘oil globules’’ of Copepoda. 
In such Copepoda as Cyclops and Canthocamptus, which appear to have 
no differentiated heart, there are always present drops of colored 
fluid, which are most numerous in well-fed and pregnant specimens. 
These drops occupy the same relative position as the blood globules 
of other Crustacea, i. e., they lie within a very thin membrane corre- 
sponding to the vascular walls of other animals. This membrane, in 
general, invests the alimentary canal, as can be very readily seen in 
the abdomen, where it inc!oses a considerable space about the intestine, 
which is filled with fluid, investing more or less completely the muscles 
