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, mamre 

 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 

 cori:)uscles, 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 ta 

 (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 CanthocampUis, 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 globulea 

 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 incloses a considerable space about the intestine,, 

 which is filled with fluid, investing more or less completely the muscles 



