Ill mode of growth and form of filament exactly like the last. I have never
been able to verify the description given by Mr. Ralfs on the authority of
Mr. Jenuer, and copied by succeeding writers, that “ the frustules stand erect
like the staves of a tub, and when dry fall down and necessarily separate.”
In the specimens I have gathered in Westmoreland, and those received from
M. De Brc'bisson, the circles are invariably arranged on a plane as in the last
species ; and although the adherence of the valves of the contiguous frustules
appears less firm than in that species, there is no lack of filaments exceeding
a semicircle, and not a few forming a complete round.
Ill my specimens the present is generally a smaller form than the last, and
the F. V. of the frustule broader in proportion to the length. The same
curious anomaly in the mode of growth is found in var. ¡i. as noted under
M. circulare, an additional proof that such peculiarities ought not to be
adopted as a specific distinction. Var. y. of our present form is probably a
sporangial state of the plant, which escaped my recognition in the freshly
gathered specimens.
Plate XXXII. 278. Var. /3. 278 ¡i. Var. y. Supp. PI. LX. 278 y.
G en u s 31. B A C IL L A R IA , Gmel.
Frustules linear, straight, united into a filament, at first attached, at
length free, coiled inwards ; valves elongate, keeled, with a longitudinal
line of puncta, keel eccentric.
The frustules of BaciUaria are those of a Nitzschia; but their
adherence after self-division into a filament, frequently of considerable
length, prevents the observer from placing them in the latter genus.
When observed in a growing state, there is another peculiarity w-hich
at once serves to discriminate our native species, and that is the
extraordinary motion with which the united frustules are endowed,
and which is not seen in any other of the filamentous Diatomaceæ.
This movement has been so well described by Mr. Thwaites in a
communication to the Linnean Society (Proceedings, vol.i. p. 311),
that I shall avail myself of his observations in the elucidation of the
subject. Mr. Thwaites remarks that, “ When the filaments have been
detached from the plants to which they adhere, a remarkable motion
is seen to commence in them. The first indication of thi.s consists in
a slight movement of a terminal frustule, which begins to slide
lengthwise over its contiguous frustule; the second acts simultaneously
in a similar manner with regard to the third, and so on
throuo-hout the whole filament ; the same action having been going
on at the same time at both ends of the filament, but in opposite
directions. The central frustule thus appears to remain stationary,
or nearly so ; while each of the others has moved with a rapidity
increasing with its distance from the centre, its own rate of movement
having been increased by the addition of that of the independent
movement of each frustule between it and the central one.
This lateral elongation of the filament continues until the point of
contact between the contiguous frustules is reduced to a very small
portion of their length, when the filament is again contracted by the
frustules sliding back again as it were over each other; and this
changed direction of movement proceeding, the filament is again
drawn out until the frustules are again only slightly in contact. The
direction of the movement is then again reversed, and continues to
alternate in opposite directions, the time occupied in passing from
the elongation in one direction to the opposite being generally about
45 seconds. I f a filament while in motion be forcibly divided, the
uninjured frustules of each portion continue to move as before,
proving that the filament is a compound structure, notwithstanding
that its frustules move in unison. When the filament is elongated to
its utmost extent, it is extremely rigid, and requires some comparatively
considerable force to bend it, the whole filament moving out of
the way of any obstacle rather than bending or separating at the
joints. A higher temperature increases the rapidity of the movement.”
The motion here so accurately described is not essentially different
from that noticeable in many of the free species of Diatomaceæ, the
peculiarity being that it is here exhibited in numerous united
frustules ; when observed in a band of one hundred or more frustules,
the singular appearances assumed by the filament under the action of
so many individuals moving at one time in apparent concert, and
another in opposition, never fail to excite astonishment.
The disposition of the coil in BaciUaria is the opposite of that in
the last genus. In BaciUaria the curve takes an upward and inward
direction, forming a roll as made by a sheet of paper, and not a
spiral as is seen in the turns of a corkscrew.
The form arises in both cases from the unusual development of the
valves, in Meridion the growth being greatest at one extremity of the