OF THE PROTOZOA. CILIATA. 295 



Trkhodina (XXIX. 15, 17), some species of Epistyl'is, ifec. — it scarcely de- 

 scribes more than one circuit round the disk, whilst in Oj^ercularia articulata 

 and EpistyVis Jiavicans it runs round the disk three times, and in other forms 

 the length lies between these two extremes. This portion consists of a double 

 row of cilia ; those of the outer row are usually somewhat shorter than those 

 of the inner, and inserted upon the ciliary disk nearly in the same line, but at a 

 different angle, as they appear to be far more strongly bent outwards. In 

 the Ycstibulum and oesophagus the cilia appear to stand in a single row. 

 The peristom bears no cilia ; those represented upon it by Stein belong to the 

 outer series of cilia of the disk, or to that portion of the spii^al which descends, 

 on the stem of the rotatoiy organ, into the vestibulimi. The latter also, 

 perhaps in conjunction with the bristle above mentioned, appear to have been 

 what induced Ehrenberg to suppose the existence of a frilled lower Up in 

 EpistyUs nutans, and Stein in all the Opercularke. 



'^ To see the particulars above described, it is peculiarly advantageous to 

 observe animals which have died during expansion." 



In Vorticella (XXYII. 1, 2, 4 ; XXIX. 1) we have a truncate anterior ex- 

 tremity, the margin of which, i. e. the peristom, is ciliated, expanded, and 

 often rather roUed outwards, and has within and rising .slightly above it the 

 rotary or ciliated disk. This is separated by a fissiu"e from the peristom 

 (XXYII. 1, 2), except at one part, where the two are continuous, and on 

 examination the disk, with its supporting stem narrowing downwards and 

 outwards obliquely into the body, appears to be a fold reflected from the 

 icner margin and surface of the peristom. The mouth opens at the bottom 

 of the fissure or cavity (the vestibulum), and is fm^nished with several cilia. 

 The ciliated disk when outstretched is elevated a little above the peristom, 

 but can be retracted and covered in by it completely. The peristom, like- 

 wise, can so cuiwe itself inwards as to include its own cilia within the ring 

 of integument which closes over, like a sphincter, the whole ciliary ap- 

 paratus of the head. The rotary disk has some general resemblance to 

 a cork or plug, which can be drawn inwards by the animalcule itself, or 

 pushed outwards, so as to serve, by its ciliated margin, to produce a vortex in 

 the fluid, and thereby fulfil the piu^DOse of a prehensile or purveying organ, 

 in addition to its locomotive power when the VorticeUa is free. The tapering- 

 basis of the disk ends below in the general cavity of the body, and is held in 

 situ by its retractor fibres, which proceed to it from the sides of the animal- 

 cule posteriorly. Its interior is continuous with the general cavity of the body. 



The unfolding of the ciliary apparatus of the head is more gradual than the 

 retraction ; and, so to speak, the animal seems to feel its way by fii\st everting 

 a portion of its delicate peristom (according to our own observation, in a sinuous 

 manner) along with a few of its stronger cilia, before expanding the rest. 



In Oplirydium (XXX. 5), the disk is rather more convex on its surface, 

 and advances somewhat higher above the peristom, but in all essential parti- 

 culars resembles that of Vorticella. On the retraction of the disk, the peri- 

 stom contracts above it into a short cylinder, and the head swells out in a 

 globose manner (XXX. 6). Between Oplirydium and Vaginicola (XXYII. 

 11) there is a close resemblance in the conformation of the ciliated organs, 

 except that, in the latter, the act of retraction agrees rather ^dth that of 

 Vorticella than with Ophrijdium. 



A truncated, thickened, somewhat everted peristom, fringed with cilia 

 (this Lachmann denies, see above), belongs to Epistylis (XXX. 11) as well as 

 to the above-named genera, and to Carchesium (XXX. 9) and Zoothamnium. 

 It has also a similar rotary disk, only rather more developed, and its stem 

 short and thick. 



