Physiology of Trichodina pediculus. 407 
seventy diameters, one may, with great facility, survey, through 
the glass sides of an aquarium, the whole body of a Hydra, 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 disk protrudes (and that only 
momentarily) beyond the plane of the vibratile crown ; 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 (6) 
is singularly irregular, especially in a transverse direction. 
A sectional view (fig. 9) presents the form of an irregular cir- 
cle with various projections, inwardly and outwardly, from 
its main course. This arises from the fact that the body 1s fluted 
and ribbed exteriorly by irregular longitudinal furrows and pro- 
jections (Pl. IX. fig. 14, 7, r), which extend from one end of it 
to the other. The ribs (r) arise with a broad expanse imme- 
diately behind the anterior ciliated margin (d'), 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 im- 
pressed 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?) about the mouth 
(m) of the vestibule (v), transforming the circular outline of the 
vibratile organ (4, b1) into a broad oval figure when this ciliated 
margin is foreshortened (fig. 13) and brought into focus with 
that part which winds spirally downwards and into (at J?) the 
aperture of the vestibule. In the form of the disk, and the cir- 
cumambient spiral vibratory crown, we are reminded rather of 
Stentor than of the Vorticellide ; nor would it be amiss to sug- 
gest here that, in this respect, Trichodina stands intermediate 
between the Vorticellidans and the group (Bursarinz) to which 
Stentor belongs. 
Owing to the presence of the reproductive organ (mn), and the 
so-called “ adherent apparatus” (fig. 10, h, 2, J, /'), 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 
