110 Dr. E. Greef on the Structure and 



peculiar nature of the anterior rotatory organ^ or the so-called 

 adoral ciliary zone, which, in the true Vorticellw, according to 

 Clapar^de and Lachmann, always forms a left-handed spiral, 

 running round the ciliary disk and then sinking into the ves- 

 tibule, which commences between the peristome and the ciliary- 

 disk. The ciliary disk, i. e. the ivhole rotatory organ, can 

 moreover be retracted into the interior of the body, and in 

 this case is closed up hy the membranous jyeristome as by a 

 sphincter or a screen. When the ciliary disk is opened out- 

 wards, it is surrounded, as if by a collar, by the peristome, which 

 IS separated from it by a more or less deep furrow, and rolled 

 outwards. The faculty of suddenly siyringing back of the 

 whole body, which contracts at the same time, and of the 

 stems, where contractile stems are present, is connected as an 

 essential character with the retractile rotatory organ. 



For the closer limitation of the Vorticellina, with regard to 

 the allied families, especially the Ophrydina^ the gelatinous 

 envelope, which occurs in the latter and is wanting in the 

 Vorticellce, has been again adopted by Stein as the distinctive 

 character between the two families, it having been already, as 

 we have seen, employed by Ehrenberg for this purpose, but 

 suppressed by Clapar^de and Lachmann. For my own part 

 I can only regret this repeated separation, as, if we leave out 

 of consideration the stem of the Vorticellina and the envelope 

 of the Ophrydina, the two families agree so closely in their 

 whole habit, structure, and vital phenomena, even to the most 

 minute points, that, in the presence of this agreement, a seve- 

 rance of the Ophrydina from the immediate alliance of the 

 Vorticellce must be regarded as forced, and opposed to the 

 natural relationship of these two groups. Probably even the 

 most experienced and careful observer would scarcely be able 

 to recognize, as such, a fissional scion of an Ophrydine (e. g. 

 Cothurnia, see PI. XII.) furnished with its posterior circlet of 

 cilia, and just escaped from the envelope of its parent — that is 

 to say, to distinguish it from the iissional scion of a true Vor- 

 ticella. It is therefore merely the exterior finger-like envelope 

 that is destined to separate the two families. But this alone, 

 as we shall endeavour to show, can not suffice for the satis- 

 factory establishment of this separation. If we could put op- 

 posite to each other the forms furnished with an envelope and 

 adherent by short peduncles, and at the same time perfectly 

 freely movable and non-pedunculate forms, such as may be 

 represented by the fissional scions or by Gerda, the separation 

 under consideration would perhajjs appear more justified. 

 But most Vorticellce are likewise attached to peduncles, which 

 on their part, again, are attached — with the exception of two 



