i I
; I;
found those which are caused by the presence of Canaliculi, to
which reference has already been made; nor must we overlook
others, which, although connected with the structure of the valve,
arise from the absence of cells. Thus in many genera the valve
is notably distinguished by the presence of a longitudinal band
of silex, free from markings, and widened into small expansions
at the centre and extremities, or at the extremities only. This
band is simply a portion of the tissue in which the silex is deposited
in a solid mass, and is probably designed to give firmness
to the valve. At the expansions of this band the silex
is still further accumulated, and forms Nodules, which also
strengthen the epidermal envelope.
That these expansions are not perforations in the valve, as
alleged by Ehrenberg, and acquiesced in by Kiitzing, might be
shown in various ways. The internal contents of the frustule
never escape at such points when the frustule is subjected to
pressure, but invariably at tbe suture or the extremities, where
the Foramina, already described, exist. Nor does the valve,
when fractured, show any disposition to break at the expansions
of the central line, as would necessarily be the case were such
points perforations, and not nodules. Moreover, the central
band of silex is itself frequently traversed by a narrow line, which
arises from the confluence of a series of cells, which thus form a
minute tube; but this tube invariably ends in a rounded extremity
at the central and terminal nodules, and does not pass
into an opening or aperture in the valve. A reference to Plate
XIX. fig. 166. will illustrate these remarks. The bending down
of this tube, and the thickening downwards of the silex at the
nodules, give the semblance of depression to the surface of the
valve at such places. But I am disposed to think that this is
merely an optical appearance; and am at all events assured that
no perforation exists at such points, and that the terms applied
to these nodules by different authors, implying that they are
openings or ostiola, are altogether inadmissible.
S e c t io n III.
I n t e r n a l S t r u c t u r e a n d C e l l - c o n t e n t s o f t h e D ia t o m
a c e o u s F r u s t u l e .
I now proceed to describe the living frustule, some of the
appearances presented by which can only be understood by a
previous acquaintance with the structural character derived from
an examination of the siliceous envelope.
A careful study of these organisms in a fresh state is necessary
to enable the observer to arrive at a correct idea of their
true character. The species are liable to material alterations
when removed for any length of time from their natural localities ;
the lapse of a few days, and sometimes of a few hours, being
sufficient to modify or altogether change the colour and arrangement
of the cell-contents, and to suspend the peculiar movements
which in many species accompany their living condition. They
must, therefore, be examined under favourable circumstances,
or our conclusions may be of the most erroneous kind ; and a
neglect of these precautions has undoubtedly led in some cases
to grave errors respecting the structure, and even the nature, of
the Diatomaceous frustule. As I look upon these organisms as
unicellular plants, I consequently regard the siliceous valves as
investing a closed membranous sac or cell. Nor is this a matter
of conjecture and analogy only. In some cases I have seen
nnmistakeable indications of the existence of this membrane,
which I have noticed to contract around the cell-contents upon
the death of the cell ; and in one species, Stauroneis pulcMla,
Plate XIX. fig. 194, in which the membrane in question appears
to possess an unusual degree of firmness, the siliceous valves, after
a slight maceration in acid, may be seen to fall away from the
internal membrane, and to leave the latter unaltered in form.
Moreover, the experiments and authority of Professor Bailey
place the existence of an internal membrane beyond all doubt.
This acute observer states that, “ If hydrofluoric acid is applied
to recent Diatomaceæ, the shell soon dissolves, leaving distinct
internal, flexible cell-membranes, retaining the general form of
b 2