STATE GEOLOGIST. 25 



tive 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. 



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 graius, 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 

 protoplasm. These are most numerous in young and well-fed ani- 

 mals, 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 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 depauperating 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 desolving of the contained 

 material is of little importance). These cells also are thus paralel- 

 lized with the " oil globules" of Copepoda. In such copepods as 

 Cyclops and Canthocamptus, which appear to have no differen- 

 tiated heart, there are always present drops of colored fluid, which 

 are most numerous in well-fed and pregnant specimens. These 



