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List of objects found in the Croton water, winter of 1881 and 1882. The specimens obtained by filtering about one barrel of water: 1. Acineta tuberosa. 2. Actinophrys sol. 3. Amoeba proteus. 4. " radiosa. 5. " verrucosa. 6. Anabaina subtularia. 7. Ankistrodesmus falcatus. 8. Anurea longispinis. 9. " monostylus. 10. Anguillula fluviatilis. 11. Arcella mitrata. 12. " vulgaris. 13. Argulus. 14.
When this has been obtained, the air disappears without our being able to discover any other reason for its disappearance than the fact that it is no longer needed. . . . If we bear these circumstances in mind, we can almost always tell whether an arcella will develop air-vesicles or no; and if it has already developed them, we can tell whether they will increase or diminish . . . The arcellae, in fact, in this power of altering their specific gravity possess a mechanism for raising themselves to the top of the water, or lowering themselves to the bottom at will.
Arcella vulgaris is a minute morsel of protoplasm, which lives in a concave-convex, brown, finely reticulated shell, through a circular opening in the concave side of which it can project itself by throwing out pseudopodia.
After from five to twenty minutes, the specific gravity of the arcella is so much lessened that it is lifted by the water with its pseudopodia, and brought up against the upper surface of the water-drop, on which it is able to travel. In from five to ten minutes the vesicles will now disappear, the last small point vanishing with a jerk.
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