vibration of these lashes in water causes the
motion, in the same way that oars propel a
boat, or—for a better comparison—as the
paddle-wings of a penguin urge it on in its
submarine chace after fishy prey. The
vibration of cilia in animalcules is sometimes
so rapid—is performed with such inconceivable
swiftness—as only to be perceptible by
the currents it produces. When, however,
the creatures become faint and dying, the
action of the cilia, then performed at a more
sober pace, is distinctly visible to the human
eye with the aid of the microscope. Now,
Ehrenberg and Kützing place the diatoms
amongst the earliest forms of animal life.
Mr. Hogg has observed a very remarkable
ciliary arrangement in many of the more
common diatoms. He has attentively watched
a diatomean moving slowly across the field
of the microscope; when, upon meeting with
an obstacle to its progress, it has changed its
course, or pushed the obstacle aside, as if
conscious of an impediment. Before satisfying
himself of the presence of cilia, he
thought the motion of these little creatures
somewhat remarkable, steering their course
as they did by a power which they were
evidently able to call into action or restrain
at will. In other organisms—the desmidaceæ
—the ciliary motion seen may be
believed to be due to a physical force acting
independently of any controlling power; in
short, the creature seems to have no will of
its own. It is a little steamer with the fires
lighted and the paddles going, but without a
crew, a pilot, or a captain. On the contrary,
with the Diatomaceæ, their cilia may be said
to act in obedience to a will; for intervals
of rest and motion are clearly perceptible.
Consequently, a diatom is an animal.

Diatoms are beautiful things to look at,
living or dead; for an unchangeable portion
of their delicate persons consists of a flinty
shield, which retains its intricate markings
and perforations after the lapse of ages—
after digestion in potent stomachs, after
burnings in fire, after boilings in acid, after
blowings about by the wind, after petrifactions
in rocks, after grindings in mills. There
are extinct and existing, as there are marine
and fresh-water species. To describe the
appearance of a diatom under a good microscope
is about as easy as to describe a veil of
Honiton lace expressly worked for a royal
bride, or to give in words a distinct idea of
the Gothic tracery to be wondered at in the
churches at Rouen and Amiens. Diatoms
are easy to find, and yet not easy to lay
hands on when found. The unskilled
manipulator may for some time endeavour to
adjust a slide, having a piece of glass exposed
not larger in size than a pea, on which he is
informed an invisible object worthy his attention
is fixed, before he is rewarded by a sight
of the Triceratium favus, extracted from the
mud of the too muddy Thames. To convey
a popular though rough notion of its appearance,
it looks like a triangular piece of what
ladies call insertion-work, of the finest
texture. The hexagonal markings of the cells
are very beautiful; and at each corner there
is a little projecting horn or hook.

Amongst the diatoms, my own favourites
are the Naviculæ, possibly because they are
my first love, never having seen a diatom
before till a charming Navicula met my
wondering gaze, and I now carry it about, as a
bosom friend, in my waistcoat pocket. Navicula
is Latin for a little ship; that is all the
mystery of its nomenclature. Look, Tom, at
this slip of glass neatly pasted over with
paper. To its centre is applied a square of
thinner glass, so that the objects are mounted
between the two glasses, and the paper is cut
away, so as to leave a transparent circle,
about the size of a fourpenny-piece. Look
sharp, Tom; what do you see within its
circumference? What, nothing? Absolutely
nothing, unless the suspicion of a
little fine dust? Observe the mark I have
made with a pencil on the paper at the edge
of the circle. Close to that we shall find
something beautiful. I slip my slide in the
microscope, and there I have it. The tiny
bark is a boat of cut rock crystal, fit to float
across a sea of light; itself might almost be
believed to be fashioned out of solidified light.
The central line must be the keel; the
translucent planking is clearly visible; and around
the sides are cut symmetrical notches, to
serve as rullocks for ethereal rowers to
navigate this brilliant gondola. What exact
Navicula this is, I know not. The slide
was sent me as a specimen of N. hippocampus,
of which, Tom, you see there
are plenty,—those long narrow transparent
Indian canoes twisted into the line of
beauty. But my Navicula belongs to none of
them; the object-mounter has given it into
the bargain, and I am very much obliged to
him for it.

Naviculæ are numerous, and widely
dispersed. The green Navicula, about the
hundredth part of an inch in length, was
found by Dr. Mantell in a pool on Clapham
Common. The golden Navicula is another
beautiful species, so named from the numerous
points within the shell giving it a bright
yellow appearance. The shell is an oblong
oval, and has upon it numerous delicate and
regular flutings. In the vicinity of Hull
many very interesting varieties of Diatomaceæ
have been found, the beauty of the varied
forms of which delight the microscopist.
It has been shown by Mr. Sollit that
the markings on some of the shells were
so fine as to range between the
thirty-thousandth and the sixty-thousandth of an
inch; the Pleurosigma strigilis having the
strongest markings, and the Navicula acus
the finest. Certain diatoms are common both
to the old world and the new. The beautiful
Meridion circulare abounds in many localities
in this country; but there is none in which