118 H- JAMES-CLARK ON THE ANATOMY 



cup nearly back to its base, and exposes the bottom of this hollow in a most convincing 

 maimer (fig. 6). In partially contracted individuals (fig. 10), the bottom of it becomes 

 elevated, and projects like a boss (c) more or less beyond the inrolled vibratile organ (b). 

 This is the condition — with the vibratory cilia more or less projecting — of those figured 

 by all observers, and especially by Stein and Busch ; and a form which the creature very 

 frequently assumes when in a confined state. 



It is an easy matter to see that their natural and accustomed shape is as we have repre- 

 sented these animals, — if one studies them undisturbed, as they creep over the body of a 

 Hydra which is attached to the side of an aquarium. With a Wollaston doublet, magni- 

 fying thirty diameters, or even a Tolles triplet, magnifying seventy-five diameters, one 

 may, with great facility, survey, through the glass sides of an aquarium, the whole body 

 of a Bydra, and watch the movements of the Trichodinas which infest it. Under these 

 conditions it is no exaggeration to say that it is very rare to meet with a Trichodina 

 whose disc protrudes — and that only momentarily — beyond the plane of the vibratile 

 crown, but on the contrary it is sunken far below this plane ; thus rendering the region 

 about this part of the body singularly transparent, light, and airy. This effect is very 

 much enhanced, moreover, by the excessively transparent, filmy exterior wall (p) which 

 projects, very prominently in profile, between the two ends of the body. 



The contour of the body behind the spiral, vibratile crown (b) is singularly irregular, 

 especially in a transverse direction. A sectional view (fig. 9) presents the form of an 

 irregular circle with various projections, inwardly and outwardly, from its main course. 

 This arises from the fact that the body is fluted and ribbed exteriorly by irregular, longi- 

 tudinal furrows and projections (fig. 14, r, r), which extend from one end of it to the other. 

 The ribs (r) arise with a broad expanse immediately behind the anterior ciliated margin 

 (d 1 ), and gradually narrow toward the mid-length, and then more gradually expand to a 

 much less width at the posterior end. At first one is impressed with the idea that they 

 are longitudinal muscles ; but, as they are more carefully examined, they do not appear to 

 be anything but mere thickenings and folds of the body-walls. 



The principal cause of the one-sidedness of the body is the protrusion of the region 

 (figs. 8, 11, 13, d, d 3 ) about the mouth (m) of the vestibule (v), transforming the circular 

 outline of the vibratile organ (I, b 1 ) into a broad oval figure, when this ciliated margin is 

 foreshortened (fig. 13) and brought into focus with that part which winds spirally down- 

 wards and into (at b 2 ) the aperture of the vestibule. In the form of the disc, and the 

 circumambient, spiral, vibratory crown, we are reminded rather of Stentor than of the 

 Vorticellida! ; nor would it be amiss to suggest here, that, in this respect, Trichodina stands 

 intermediate between the Vorticellidans and the group (Bursarinse) to which Stentor 

 belongs. 



Owing to the presence of the reproductive organ (u), and the so-called " adherent appa- 

 ratus" (fig. 10, h, i, I, I 1 ), the expanded circular base is even more conspicuous than the 

 discal end. It most frequently presents itself as a rather abruptly widening, perfectly 

 circular, disciform expansion, whose plane trends transverse to the axis of the body. It 

 varies in form more or less according to the surface over which it is creeping : at one 

 moment sunken (fig. 14) like a cast into a depression of the body of the Hydra, and at the 

 next instant assuming the reverse form (fig. 10), and embracing some projecting group of 

 cnidse, or as it were wrapped around the parietes of an extremely elongated tentacle. As 

 a farther extension, the base is margined by an annular membrane, or velum (Z,/ 1 ), and 



