Physiology of Trichodina pediculus. 411 
tip of the bristle were sweeping along the row of cilia and 
pushing them back in succession. In addition to this, it will be 
noticed that the end of this false bristle varies in thickness from 
moment to moment during the shifting of the pomt of coin- 
cidence ; and fina!ly it may be remarked that it frequently seems 
to be broken into a series of dots, or short irregular pieces. 
This last feature gives the clue to the mystery. The apparently 
disjointed pieces of the tip of the false bristle are nothing more 
or less than the foreshortened points of the closely approxi- 
mated successive cilia as they project towards the eye during 
the descent of the row into the vestibule. The point of coin- 
cidence mentioned above is the place where the row bends 
abruptly towards the aperture of the vestibule ; and the shifting 
of this pot is the changing of the trend of the ciliary tips. 
The line of attachment of the cilia is not changeable, and it may 
be readily traced to the bottom of the vestibule ; but the cilia, 
whilst projecting at various and constantly diversified angles 
from their base of attachment, are so disposed that their ap- 
proximated tips form a frequently varymg undulating line. 
That the “ bristle ” sometimes unaccountably disappears 
during observation, arises from the fact that the cilia have so 
changed their position that they do not afford a view which 
presents the appearance of such a body. Usually, however, the 
cilia are curved transversely to the axis of the vestibule, so that 
they form as it were a cylinder of juxtaposed hoops or circles ; 
and it is not to be wondered at, therefore, that in almost any 
position the outline of this cylinder should appear as a single 
line or filament. In a view directly into the aperture of the 
vestibule the bristle, so called, is not to be seen, for the very 
reason that the cylinder is presented endwise; and on this 
account, too, the vestibule appears to have a double contour, the 
inner one of these contours being nothing less than the series 
of curved cilia placed closely side by side and trending trans- 
versely to the axis of the cavity in question. This is a particu- 
larly facile observation in Vorticella, and none the less so in 
Carchesium. Finally, it may be said on this pomt—and, coming 
last, it is of no less importance than what has preceded, but, on 
the contrary, is worthy of the utmost consideration in an optical 
point of view—that were the so-called bristle a genuine body 
it would be in focus at only one particular adjustment of the 
lens ; whereas we find that, having obtained what appears to be 
a clear and definite view of a filament, it does not go out of view 
by a change of the focus over a considerable extent above or 
below that horizon. This, one may readily perceive, would be 
the case in observing the outline of a transparent cylinder ; and 
as the closely approximated curved cilia form such a cylinder, 
