Phystology of Trichodina pediculus. 405 
flexibility, and an unlimited adaptability to whatever surface it 
may come upon, no matter how uneven it happens to be. The 
intimate structure of the chitmous ring does not interfere in the 
least with, but on the contrary appears to assist in, the flexures 
of the base. The latter is always the point of attachment ; and 
upon this part of the body the animal may be seen, almost at all 
times, gliding to and fro like a miniature cup (figs. 1, 2), now 
on the upper side of a Hydra, and then on the lower side. At 
one moment several individuals are crowded together on a 
tentacle, and in the next instant scattered along its length 
from base to tip, and giving to it a singular, irregular, changeable 
outline. At times the Hydra seems to be strangely knotted, 
and ungainly in outline, when, upon close examination, we 
ascertain that it is crowded with a swarm of Keronas, upon 
several of whose convex backs one, two, or three Trichodinas are 
seated, enjoying the pleasure of locomotion without the effort 
of producing it. Not unfrequently an individual may be seen to 
leave its reptant mode of progress and take to the surround- 
ing element. Then it swims, at times very swiftly, either in a 
fully expanded state, or half expanded (fig. 4), or even shortens 
its length so much that its body resembles a wheel (fig. 5) 
rolling on its axis, or turning end for end and performing a 
series of somersaults with great rapidity. Presently it returns 
to its more quiet mode of life, sliding spectre-like over the 
animate surface which forms its principal field of operations. 
During its act of reptation it revolves very slowly upon its lon- 
gitudinal axis, as if upon a pivot, and most frequently, if not 
always, wheels to the right. 
§ 2. Specrric ReLatronsuip.—When looking at perfectly 
fresh and lively specimens of this Infusorian, one can hardly 
believe, at first, that their deep, cyathiform, dicebox-like bodies 
(figs. 1, 2) are specifically identical with the straight and 
broad cylindrical forms which are figured by Ehrenberg and 
Dujardin, or with the turban-shaped bodies which are illus- 
trated in the papers of Stein and Busch; but when, upon pro- 
longed investigation, we see that the least interference with their 
freedom of motion causes them to assume a depressed form and 
a partially retracted margin, we recognize their close resem- 
blance, at least, to those of the above-named authors. The 
former state represents nature in reality; the latter exhibits her 
in a disguised shape. It is therefore with no small degree of 
reluctance that one concludes to identify the flexible, irregu- 
larly funnel-shaped, conspicuously asymmetrical body of the Ame- 
rican Trichodina with the seemingly stiff, precisely outlined, 
cylindrical or conical figures illustrated in European works ; 
but a careful study of this under various conditions, both in re- 
