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In the course of some unpublished experiments on the rôle of acid in protozoan digestion, I had occasion to use coagulated white of egg as a food for certain colonial Vorticellidæ. This substance, diluted with water before coagulation, and therefore given to the animals as a finely granular precipitate, is usually found after enclosure in the form of smooth oval or spherical masses (Plate 34, fig. 2,C .s .), which are widely different both in refractive characters and in size, from the minute irregular particles offered for ingestion. So striking is the contrast, and so constant is its occurrence with this form of food, that I attempted to watch the process by which the spherical ingesta are shaped: in this attempt I was struck by the clearness with which some characteristic features of the process are demonstrable inCarchesium , and realize that the very act, with the performance of which I was especially concerned, was apparently bound up with the operation of a mechanism undescribed in existing descriptions of digestion in the Protozoa. This led me to record, in the paper which follows, the result of sequent observation of the phenomena of ingestion, digestion, and ejection inCarchesium ; for, while the intracellular solution of food is as trulywithout the cell substance as in the case ofAmæba ; while, indeed, there is no fundamental fact established for the digestion of that animal which does not find a parallel here —the digestive cycle is shortened, unfamiliar details of the process are striking, an even the more familiar events occur with a dramatic vividness which makes them almost strange. The animals I watched (Carchesium andEpistylis ) were kept in hanging drops of water, and were thus under no unusual pressure. Various solutions and suspended particles were added at different times to the water of these hanging drops, and, in order to examine the results of the addition of each substance, I used, as a rule, Oc. 3, Obj. F. (Zeiss) or Oc. 3, Obj. 1/12 oil immersion (Zeiss or Leitz), working sometimes by daylight, sometimes by artificial light. The focal length of these objectives does not allow the examination of animals ill any great depth of encircling fluid; but such signs of lesion as extreme convexity of the peristome, the development of an enormous contractile vacuole, definition of the outline of the nucleus, are so prompt in their appearance in these Vorticellidæ, and so unmistakable, that it is possible, by the use of recurrent immersions in abundant water to watch the same colony for many successive days.

philosophical transactions of the royal society of london. b

IX. On the constitution and mode of formation of "food vacuoles" in infusoria, as illustrated by the history of the processes of digestion in carchesium polypinum