the group as
a whole exhibits a remarkable uniformity of structure. The Bacillarieae
is one of the large groups of Algae, placed by some in close proximity
to the Conjugatae and by others as an order of the Brown Algae (or
Phaeophyceae), but their characters are so distinctive and their
structure is so uniform as to warrant the separation of the diatoms as a
distinct class. The affinities of the group are doubtful.

The diatoms exhibit great variety of form. While some species are
circular and more or less disk-shaped, others are oval in outline. Some
are linear, as _Synedra Ulna_ (fig. 2), others more or less crescentic;
others again are cuneate, as _Podosphenia Lyngbyii_ (fig. 3); some few
have a sigmoid outline, as _Pleurosigma balticum_ (fig. 4); but the
prevailing forms are naviculoid, as in the large family Naviculaceae, of
which the genus _Navicula_ embraces upwards of 1000 species. They vary
also in their modes of growth,--some being free-floating, others
attached to foreign bodies by simple or branched gelatinous stalks,
which in some species are short and thick, while in others they are long
and slender. In some genera the forms are simple, while in others the
frustules are connected together in ribbon-like filaments, or form, as
in other cases, zigzag chains. In some genera the individuals are naked,
while in many others they are enclosed in a more or less definite
gelatinous investment. The conditions necessary to their growth are
moisture and light. Wherever these circumstances coexist, diatomaceous
forms will almost invariably be found. They occur mixed with other
organisms on the surface of moist rocks; in streamlets and pools, they
form a brownish stratum on the surface of the mud, or cover the stems
and leaves of water plants or floating twigs with a furry investment.
Marine forms are usually attached to various sea-weeds, and many are
found in the stomachs of molluscs, holothurians, ascidians and other
denizens of the ocean. The fresh-water forms are specifically distinct
from those incidental to salt or brackish water,--fresh-water species,
however, are sometimes carried some distance into the sea by the force
of the current, and in tidal rivers marine forms are carried up by the
force of the tide. Some notion may be formed of the extreme minuteness
of these forms from the fact that one the length of which is 1/200th of
an inch may be considered as beyond the medium size. Some few, indeed,
are much l