406 Prof. H. James-Clark on the Anatomy and 
gard to space for movement and the quality of the water, in- 
evitably leads to the conclusion that the European figures repre- 
sent the creature in an abnormal, or at least a more or less re- 
strained condition, certaimly not in a perfectly healthy state. 
If a Hydra, upon which some of these animals are living, is 
transferred to a flat watch-glass, and the water is frequently re- 
newed, there is not the least difficulty in studying this In- 
fusorian whilst in its fullest degree of expansion, and even with 
a magnifying-power of at least five hundred diameters. In fact 
it is absolutely necessary that the body should be fully expanded, 
in order to understand the relation and nature of certain parts 
of its organism—especially the vestibule and cesophagus, and tlie 
contractile vesicle. Ina semiexpanded state of the body these 
parts are confused, and it becomes impossible to ascertain their 
character with even the least degree of satisfaction. It is on 
this account that neither the figures of Stein nor those of Busch 
give the faintest idea of what the anterior region of Trichodina is 
like; and we actually get a better and truer impression of its 
character from the almost forgotten illustrations of Ehrenberg 
than from the more modern and what ought to be more correct 
delineations of this animal. 
§ 3. Form.—The form of the body is like that of a heavy 
wine-glass (figs. 1, 2, 8, 14) with a very thick and but slightly ex- 
panded base. The plane of the margin of the front, 2. e. the 
peristome (d'), lies parallel with that of the base, or “ adherent 
organ,” and nearly at right angles to the axis of the body. The 
disk (c, c1), or area encompassed by the vibratory crown (8), is 
deeply depressed, so that the anterior end of the body, not only 
externally but internally, is truly cyathiform. In fully expanded 
individuals the depression of the disk extends nearly to half the 
depth (at ce) of the body, and occupies at least nine-tenths of the 
diameter of this region. At times the animal suddenly recurves 
the edge of the cup nearly back to its base, and exposes the 
bottom of this hollow in a most convincing manner (fig. 6). _In 
partially contracted individuals (fig. 10), the bottom of it be- 
comes elevated, and projects like a boss (ec) 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 represented 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, 
magnifying thirty diameters, or even a Tolles triplet, magnifying 
