maintains a connection with a foreign body ; we have this form
in Synedra and Gomphonema, &c. In others, again, it is enclosed,
with mnltitudes of its confrères, in a gelatinous or membranaceous
thallus, as in Schizonema, Encyonema and their allies, and
the aggregation of the frustules in such cases sometimes rivals
in form and size the branching filaments of the larger Algæ.
Upon submitting living Diatomaceæ to a microscopic examination,
their forms are found to be exceedingly diversified, though
the mineral nature of their epiderm confers upon them a general
character which reminds the observer of inorganic ratber than of
vegetable or animal productions. Thus prismatic and quadrilateral
forms are among the most general ; linear, crescent, or
boat-shaped outlines are not unfrequent ; while indidivnals gracefully
curved in a sigmoid manner, and others resembling miniature
shields or circular discs, occasionally present themselves.
S e c t io n II.
S t r u c t u r e o p t h e S i l ic e o u s E p id e r m o p t h e D ia t o m a c e o u s
F r u s t u l e .
To ascertain the true form and structure of the Diatomaceous
frustule requires a careful microscopic examination, not only
of living, but of desiccated or prepared specimens. Indeed it
is only after an examination of the latter, that the true character
of the living frustule can be understood. I shaU therefore
commence my description of the structure of these organisms
by a reference to frustules which have been deprived of their
softer parts, either by submitting them to the action of fire, or
to the corrosive infinence of an acid.
The epiderm of the Diatom consists of two siliceous plates or
valves, usually of the most perfect symmetry. When first produced,
these valves are closely applied to each other, and the line
of junction forms a suture along which the valves readily separate
during the process of self-division which speedily follows the
perfect formation of the cell. It seems to be a law with these
organisms, that no portion of the internal cell-membrane can be
exposed to the free action of the surrounding water, without
secreting a siliceous epiderm ; the moment the valves become
separated in the process of self-division, we consequently find
that the secretion of a third plate of silex commences. This
plate forms a band between the valves, and will for convenience
sake be afterwards spoken of as the Connecting Membrane.
As self-division is continually going on while the frustules are
in a healthy or growing state, it is rare to find a specimen in
which the valves are not in some degree separated, and consequently
in which there is not more or less of a connecting
membrane.
But that no such membrane exists on the first production of
the frustule is obvious, upon the examination of a specimen in
which self-division is just completed, as in PI. VIII. fig. 59 if •
PI. XVIII. fig. 167 d ; and PL XXVII. fig. 335 d, compared
with PI. VIII. fig. 59 h ; PL XVIII. fig. 169 b ; and PL XXVII.
fig. 235 b, in which the process has made some advance. The
process of self-division wiU be described hereafter ; the present
remarks are intended to show that the Epiderm of the Diatom,
strictly speaking, consists of only two parts, the connecting
membrane being an addition consequent upon the development
which attends the growth of the frustule.
Along the line of suture in disciform or circular frustules, but
more generally at the extremities of the valves only, when the
Diatom is of an oblong, linear, or elongated form, there exist
perforations in the silex, which permit the surrounding water to
have access to the surface of the internal cell-membrane. The
formation of silex seems occasionally to be arrested in the neighbourhood
of these spots, and the connecting membrane is in
consequence either wholly or partially interrupted at such places.
Thus, after the internal cell-membrane is removed by acid, when
it often happens that the valves fall away from the connecting
membrane, the latter separates into two parts, and the frustuD
has in consequence been described as consisting of four plates.
The interruptions in the siliceous epiderm, to which reference
has just been made, are usually apparent as slight depressions
at the extremities of the frustule; and the appearances they
present have been denominated “ pnnctn ” by Mr. Kalfs. In