Physiology of Trichodina pediculas. 403 
covered fact the naturalist takes one step higher on the hill of 
science, whilst in the latter he is for ever trying to get the first 
foothold in the ascent. 
Of all the Protozoa, there are none which have so great a claim 
upon the naturalist’s time for investigation as the Vorticellide. 
The want of a precise understanding of their structure led, in 
the first place, to their being classed with the Zoophyta, and 
(simply on account of their similarity in form) among the Hydras. 
This was the first retrocession. After Ehrenberg had promul- 
gated the opinion that they possessed a distinct intestine, whose 
two ends approximated each other, we find Van der Hoeven, in 
the second edition of his ‘ Handbook of Zoology,’ comparing 
them to the Bryozoa, and avowing his belief that their future 
place will be among the lowest groups of Mollusca. Here we 
have a still deeper plunge into the vortex of confusion—not so 
much, however, if at all, to the discredit of the Dutch natu- 
ralist as to that of those who came after him. The apparent 
similarity of the organization of the Vorticellide to that of the 
Bryozoa was no small warrant for his suggestion ; but after almost 
every microscopist of any degree of reliability who looked at 
these infusorians had disproved and denied the presence of the 
intestine so elaborately set forth by the Berlin micrographer, 
and nothing was left but a mere resemblance in outward form 
to the Bryozoa, it was, to say the least, a very - far-fetched 
comparison when Professor Agassiz homologized them with the 
Mollusca, declaring that he had satisfied himself of the ‘‘ pro- 
priety of uniting the Vorticellide with Bryozoa.” 
Ere this, too, Lachmann (Mill. Archiv, 1856) had shown 
that the whole group of ciliated Infusoria possess a conformity 
of organization altogether unlike that of any other. The pro- 
found researches of this early-lamented observer left no doubt 
as to the dissimilarity between the Vorticellide and Bryozoa. 
Here was, at last, a step taken in the right direction; and when 
this author, in connexion with Claparéde, published the ‘ Etudes 
sur les Infusoires et les Rhizopodes,’ the climax of proof was 
attained in the abundance of details presented in that remark- 
able volume. Among the many questions which are discussed 
in that work, that of the unicellularity of the Infusoria receives 
a considerable share of attention ; and a decided ground is taken 
in favour of their pluricellularity—not so much, however, on 
account of their being known to consist of more than one cell 
as of the fact of their possessing such a variety of organs and 
performing so many diverse functions. 
The greatest variety of this kind is most elaborately exempli- 
fied by the group of Vorticellide; but yet it rises, from the 
lowest of the class, through such imsensible grades, that the 
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