tincture of iodine causes tlie internal membrane to contract
upon the cell-contents, and converts these, from the golden yellow
which they exhibit in some species, into a bright green ; and
that a weak solution of sulphuric acid, while it effects the same
contraction in the cell-wall, gives to the contents, which have
been previously treated with iodine, a dark brown hue.
Alcohol, on the other hand, as in the case of vegetable cells
in general, dissolves the utricle and its contained endochrome,
or, at all events, entirely removes their colour, and leaves their
siliceous epiderm in a state of perfect transparency. It does
not, however, dissolve the envelope in which the frustules of the
frondose forms are imbedded, nor the filamentous stipes or
gelatinous cushions to which other species are attached.
S e c t io n IV.
M o v em e n t s o e t h e D ia t o m a c e æ .
One of the most striking circumstances connected with the
living frustule, is the singular motion which most of the free
species exhibit.
This motion is of a peculiar kind, being generally a series of
jerks, producing a rectilinear movement in one direction, and a
return, upon nearly the same path, after a few moments’ pause,
by another series of isochronal impulsions.
The movement is evidently of a mechanical nature, produced
by the operation of a force not depending upon the volition of the
living organisms. An obstacle in the path is not avoided, but
pushed aside; or, if it be sufficient to avert the onward course
of the frustule, the latter is detained for a time equal to that
which it would have occupied in its forward progression, and
then retires from the impediment, as if it had accomplished its
full course.
There is certainly no character of aiiimality in the movement;
and the observer familiar with the phaenomena of life in the earlier
stages of vegetable existence, is constrained to seek a counterpart
in the involuntary motions of the filaments of the Oscilla-
toriece, or of the gemmiparous spores of the P u d and Conferva.
The movements of the Diatomaceæ appear rapid and vivacious
under the microscope ; but it must be remembered that the
high powers usually employed in the observation of these minute
organisms magnify their motions as well as their bulk. I
have noted the movements of several species with the aid of an
eye-piece micrometer and a seconds watch, and found that one
of the most rapid, viz. Bacillaria paradoxa, moved over -guott
of an inch in a second ; Pinnularia radiosa, one of the slowest,
over -siinith of an inch in the same time ; and that the same
period was occupied by Pinnularia ohlonga in traversing aifootli
of an inch, Nitzschia linearis -g-jVotli of an inch, and Pleurosigma
strigosum ^oTitb of an inch. Or, expressing the spaces and
times by other units, we find that the most active required somewhat
more than three minutes to accomplish movements whose
sum would make one inch, and the slowest nearly an horn- to
perform the same feat.
These movements are usually noticed only in the free species
of the class, as in those belonging to the genera Navicula, N itzschia,
and others, and especially in the more minute or younger
individuals. Motion is not, however, confined to these, but may
at times be detected in other forms, and even the frustules of
attached species, as those of Gomphonema, when forcibly separated
from their stipes, occasionally exhibit an evident tendency
to change their position. The movements in the latter are, however,
exceedingly languid, and have nothing of the isoclironism
so notable in the others.
Of the cause of these movements I fear I can give but a very
imperfect account. It appears certain that they do not arise
from any external organs of motion. The more accui’ate instruments
now in the hands of the observer have enabled him confidently
to affirm, that all statements resting upon the revelations
of more imperfect object-glasses, which have assigned
motile cilia, or feet, to the Diatomaceous frustule, have been
founded upon illusion and mistake. Among tbe hundreds of
species which I have examined in every stage of growth and
phase of movement, aided by glasses which have never been sm--
passed for clearness and definition, I have never been able to
detect any semblance of a motile organ ; iior have I, by coloin