OF THE PROTOZOA.- — CILIATA. 341 



for the phenomenon. This last view comprehends the interpretation Stein 

 puts upon the movement in question in the Protozoa, which is, that the chlo- 

 rophyll-globules by their action on light, by the exhalation of carbonic acid 

 gas, and the resultant chemical forces developed, produce the revolving move- 

 ment ; for, as he remarks, the movements of the animals have nothing to do 

 with the rotation, as some have suggested, seeing that it goes on \^hcn 

 they are in perfect repose ; and moreover is seen only in those rich in chlo-' 

 rophyll, and not in colouiiess individuals. 



In elucidation of chemico-vital action as a motor force, we may allude to 

 vegetable physiology, which teaches us its power in the circulation of the sap 

 through the appointed channels in the leaves and thence downwards through 

 the inner bark. But, apart from the influence of chemico-vital forces, we cannot 

 exclude the idea that the propulsive force of the oesophagus, in impelling food 

 or water into the general cavity, must aid the current, even if its axis do not 

 precisely correspond with the course at the point where it is first operative, 

 since, from the difference in the arcs described by the course of the stream 

 and by the oesophageal current, the two must eventually become coincident 

 and concurrent. 



In a recent letter to us. Dr. Strethill Wright remarks that " in Carche- 

 siuni jpolypinum active molecular movements may be detected throughout 

 every part of the zooid (animalcule), even in the thickened rim upon Avhich 

 the cilia are placed. This movement seems to be distinct from the rotatory 

 motion of the whole contents of the body, so readily seen in Epistylis grandis, 

 and which only occasionally occurs in Carchesium. The zooids of the class 

 of Protozoa seem to be composed of sarcode in its most fluid state, enclosed 

 in a delicate contractile coat. In this sarcode a desultoiy circulation occurs, 

 either as molecular motion or as steady rotation, or as a backward and for- 

 ward flowing occasioned by change of shape in the body, as in Ojjhr'i/dium 

 versatile.^' 



The Encystestg-peocess en" the Cill\ted Protozoa (XXYIII. 6, 7, 66, 

 67, 74-76 ; XXIX. 18, 19, 21-23, 39-46, 52-58).— Although the encysting- 

 process is very frequently associated with the act of reproduction, yet it is 

 also concerned 'udth the preservation of individual life, and, so far, deserves 

 consideration apart from the former. Were it not for some provision against 

 such a contingency, animalcular life would be exposed to wide-spread destruc- 

 tion by the change of seasons, by the drying up of the pools and ditches they 

 inhabit, and by other injurious external influences. Such a provision is made 

 by the act of encysting, which enables these minute animal organisms at all 

 ages to resist those destructive agencies, and also provides for their almost 

 imlimited diffiision. The constniction of sheaths around animalcules is an- 

 other protective act (see p. 282), but difl'ers from encysting in not completely 

 enclosing them. 



When an animalcule is about to encyst itself, its movements become less 

 active, and presently cease ; at the same time it withdraws and folds up its 

 rotary or other prominent process, closes its oral aperture and contracts itself 

 in a more or less spherical shape, and its cilia disappear. Having proceeded 

 thus far, an excretion is thrown out around, which gradually hardens, assumes 

 a membranous form, and invests the animalcule as a cyst or case. It may 

 happen that the construction of the cyst commences before the animal is 

 quiescent, while it still moves slowly about or revolves on itself by the out- 

 pouring of the soft gelatinous matter out of which it is to be elaborated, as 

 is seen in Amphileptus (XXIX. 19), Colpoda (XXIX. 35—43), and Chilodon 

 (XXIX. 48-58). Moreover, after the animalcule is enclosed within its case, 

 it may for a time vary its figure, and also turn on itself with more or less 



