OF THE PROTOZOA. CILIATA'. 843 



movements of the animalcule become slower, and before ceasing altogether, 

 consist in a simple rotation without change of position. The cilia are next 

 seen to become indistinct and to disappear ; and a delicate line, removed some 

 little distance from the peripheiy of the enclosed animal, makes its appear- 

 ance, indicating the limit of a soft gelatinous envelope. Whilst this proceeds, 

 the animal assumes a more globular and contracted figure, chiefly by folding 

 do^^-n its lip- or trunk-hke process upon its general sui'face. The secreted 

 covering in the meanwhile gains in firmness, but loses in thickness, and thus 

 acquii'es the character of a membrane, which closely invests the Trachel'mSy 

 except at places Avhere the two surfaces are separate and distinct. 



This may be termed the fii^st degree of encysting, and affects the creature 

 so slightly that it can shake off its coating of its own accord, and, by rupturing 

 its sac, reassume its pristine appearance and activity. This phenomenon was 

 witnessed four times in the same individual by Cohn, and supposed by him 

 to have been induced by the abnormal conditions (the action of light, &c.) in 

 which the animalcule was placed under the microscope. Stein {op, cit. 

 p. 133) in a similar manner recounts the formation of a cyst around Chilodon 

 Cucullulus, and the possibility of setting it free by breaking down the cyst by 

 pressure. In Trachelius the development of the cyst, to the stage described, 

 occupied, according to Cohn, only ten minutes. ^Tiere the i)rocess advances 

 beyond this degree, the cyst commonly acquires a denser and firmer con- 

 sistence ; the animalcule can no longer deliver itself at once of its own accord 

 from its prison, but undergoes a further change from its normal form, and 

 requires those vivifying influences of external warmth, light, and moisture, 

 such as spring-time brings with it, to arouse it from its torpid state, and to 

 cause the reappearance of its hitherto obliterated organs. 



Stein has very copious details of the whole process of encysting in various 

 Ciliated Protozoa ; but in none is that process more interesting to follow than 

 in the Vorticellma. In members of this family the state of extreme contrac- 

 tion, induced by some external cause obnoxious to them, becomes fixed, and 

 only the irregularly- curved space covered over by the completely-closed 

 peristom indicates the complicated ciliary apparatus of the head ; and even 

 this decreases to a streak, and at length vanishes altogether. Whilst this 

 goes foi-ward, a membrane forms around the being which is now detached 

 from its stem, and a globular or ovoid cyst, containing a nucleus and a con- 

 tractile vesicle, is the representative of the once active and elaborately- 

 organized Vorticella. 



To what degree the encysting process may advance without depriving the 

 animal of its ability to recover its freedom and original character, is well ex- 

 emplified by Auerbach's observation on the cysts of Oxytricha Pellionella 

 (XXIX. 21, 24) (Zeitschr. 1854, v. p. 430). This able microscopist found a 

 number of globular cysts, with two coats, enclosing a homogeneous, finely- 

 granular, brown substance, within which was a darker, rounded body (XXIX. 

 21), or at times two, and more rarely three such, seemingly derived from it, 

 indicating the nucleus. The contents naturally filled the capsule ; the addi- 

 tion, however, of a little mmiatic acid caused them to shrink into a roimdish 

 body, somewhat more extended on one side, and traversed by a few deep 

 folds or fissures (XXIX. 22). Such were the bodies met with during the 

 continuance of winter ; but when early spring arrived, these began to exhibit 

 signs of vital activity within. 



The fii'st change remarked was the appearance of a vesicle, which by 

 degrees acquired increased contractility ; then the body retracted itself from 

 the cyst-wall and commenced to revolve in a vacillating manner, whilst the 

 outer granular lamina of the cyst broke away. Cilia now could he seen dis- 



