STRUCTURE OF THE BODY. 259 



the rest of the surface of the body, but leading so gradually into the 

 mouth that it seems to represent only a sort of pre-oral cavity. 



The Peristome (Fig. 130), as seen in these constantly rotating 

 animals, usually appears as a conical shell, with a slit running up 

 its whole length, opening to the exterior at the right side of the 

 anterior pole, and thence extending obliquely towards the median 

 plane. The narrowed posterior end is continued further into a 

 short gullet, which is never absent (as Stein alleges), but can be 

 followed plainly to the anterior limit of the medullary substance, into 

 which it distinctly sinks. The more or less widely gaping slit of the 

 peristomial funnel, whose left border is beset by the adoral cilia, 

 belongs to the ventral surface. 



These conditions are entirely altered when one has the opportunity 

 of observing the animal feeding. In this act the peristome (Fig. 127) 

 broadens out by the unfolding of its lateral borders into a triangular 

 space, which is recognisable as the anterior portion of the ventral 

 surface obliquely truncated in a dorsal direction. The space is, more- 

 over, almost symmetrical, 1 which shows that the left peristomial 

 border exceeds the right in length and mobility, and therefore projects 

 further out when the funnel is unfolded. The surface is by no means 

 perfectly even, but sinks in posteriorly, so that it passes without any 

 well defined boundary line into the above-mentioned gullet. One 

 sometimes sees these animals creeping on their expanded peristome, 

 as snails do on their foot, without in any way changing the position 

 of the body. The adoral cilia keep up a lively motion, which some- 

 times looks like the rapid rotation of a wheel. The other cilia round 

 the border are stronger and longer than those of the general body 

 surface. On the other hand, the peristomial region itself is (according 

 to Stein) destitute of cilia. 



Besides the gullet (oesophagus) which runs obliquely through the 

 cortical layer, we find internally a nucleus and contractile vacuoles, 

 both of which, of course, belong to the ectosarc. 



The Nucleus lies on the ventral surface, above the middle line, 

 sometimes far forward, sometimes posteriorly. It is only faintly 

 defined, pale, and homogeneous, extended, and not straight, but kidney- 

 shaped. The median constriction, which Loven and Wising thought 

 they perceived, does not in reality exist. Nor have I been able to 

 find a nucleolus, although Wising reports having several times ob- 

 served it as a small round body beside the nucleus. Stein also has 

 looked for it in vain. 



1 Deceived by such appearances, I have, in the first edition of this work, credited the 

 so-called Paramcecium coli with a terminal mouth, and have suggested its association with 

 the genus HolopJirya, if, indeed, it were not preferable to erect it into a new genus. 



