OF THE PROTOZOA. CILIATA. 291 



Vorticella, Carclies'mm, and Zoothamnium, and the lowest in the nearly sessile 

 VaginicoJa, and in the rigid-stalked EpisttjUs. The movements of the stems 

 of Vorticelhe are most astonishing by their activity and energy. In their 

 contraction, which is much quicker than extension, the pedicle is t^\dsted into 

 a close spii-al comparable to a coiled spring ; and besides this action, by which 

 the animal is instantaneously drawn do\^Ti to the point of attachment, the 

 body itself shortens, and the ciliated head and appendages are retracted under 

 cover of the general integument. The branched pedicle of Zoothcnnn'mm is 

 less actively contractile, although still capable of considerable movement, 

 whilst that of Opercularia and Epistylis is quite rigid, or very slightly 

 flexible, and this in most species only in younger stems, before they are in- 

 durated by age. In Opercularia herherina we have the most marked example 

 of flexibility of the stem among rigid-stalked genera. 



Apart from the movements of the animacules dependent on their pedicles, 

 others are due to the contraction and elongation of their bodies, and to the 

 retraction and extension of their rotarv apparatus. In the instance of Vagi- 

 nicola (XXYII. 11), of CotJnmiia (XXX. 12, 13, 14, 15), and of Tintinnm, 

 these, indeed, are the only movements of which those genera are capable, — the 

 external sheath constituting of itself a safe house of defence when the ani- 

 malcule retreats within it, and thiLS ofl'ering a compensatory provision in lieu 

 of the locomotive power of the freely- swimming Cihata, or of the actively- 

 coihng spiral of Voi^ticeUa. On the other hand, when not in retreat, the ani- 

 malcule outstretches itself, and, advancing its ciliated delicate head beyond 

 the hmit of the case (XXYII. 10, 11), expands its ciliary apparatus. 



The animalcules fi^ed on rigid stems appear exposed to every passing 

 danger without defence ; natiu'e, however, has fiu-nLshed them with a firm 

 resistant integument within the anterior margin or peristom, of which they 

 can completely retract the delicate rotary disc and ciliated head. However, 

 they are not positively motionless ; for a certain latitude of motion is allowed 

 them by their mode of articulation, and by the annular segmentation of the 

 posterior extremity (XXYII. 16), in addition to the possible contraction into 

 an ovoid or more or less globular figure. In Opercularia herheriformis the 

 contraction of the body is facilitated by the transverse rugae which normally 

 exist, — ^whilst in Oplirydium it is carried so far that the elongated figure be- 

 comes oval, and, the head being retracted, the animal presents itself as an 

 inconsiderable prominence above the sui'face of the gelatinous mass it rests 

 upon. The absence of a protecting sheath in this genus is partly compen- 

 sated for, fiu'ther, by the aggregation of the Ophrydia, since the globose mass 

 produced is of itself a security, and is rendered still more so by its revolving 

 movements, the result of accidental external forces, and, we may suppose, 

 also by the activity of the animals projecting from its surface. 



The VorticeUina and Oplirydina live as free beings for a certain time after 

 their production, whether by fission or by gemmation, or by internal germs 

 or embryos. In the case of the products by gemmation and fission, this 

 locomotive power is due to the temporary formation of a wreath of cilia be- 

 hind the posterior third of the body, as mentioned in a preceding page ; and 

 it is curious that it is not then the head which moves in advance, but the 

 hinder extremity, by which attachment is to be presently made. There 

 seems to be an object in this backward progression ; for by it the animal is 

 brought directly into contact with any object to which it can affix itself, and 

 its attachment made more fii'm. The part to be attached is the first to come 

 into contact with the supporting medium ; and whether it proceeds to secrete 

 about itself a sheath, or to develope a peduncle, it finds itself rightly placed 

 without any revolution of the body. 



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