satisfied, but which increases with the supply
of food.

In size, the Infusories range between the
extreme dimensions of from one to three
millimetres, on the one hand, for the very
largest—a millimetre is, approximatively,
rather less than the twenty-fifth part of an
English inch—and of a thousandth part of
that size for the smallest. The mean of their
magnitude is from one to five-tenths of a
millimetre. The largest appear to the naked
eye like white or coloured points attached to
divers submerged bodies, or like fine dust
floating in the liquid. The rest are only
visible by the aid of the microscope. They
are almost all semi-transparent and colourless;
several are green; a few are blue or
red: a few others are brown or blackish.
The simplest forms of animal life, the
protozoa, that is the Rhizopod group, are
composed merely (at least in appearance), of a
fleshy, homogeneous, glutinous substance,
called sarcode, which has no visible organs,
but which is nevertheless vitally organised,
since it has the power of contracting and
putting forth extensions indefinitely in
various ways and directions—since, in short,
it is endowed with life. No further proof is
needed of the extreme simplicity of organisation
of these. Still, even when they have no
integument proper, it is not meant to deny
that they have a surface, like that of flour-
paste or thin glue, when it has cooled to a
jelly. To the company of such humble
fellows as these, the sponges have been
degraded, in spite of their size and their
apparently elaborate structure.

More complex genera and species of real
Infusories, as the Parameciums and the
Lacrymarias, have distinct outer coats to protect
them, which often make them look as if their
bodies were contained in a hairy and purse-
like net. Their means of locomotion are
either actively-vibrating cilia (i. e., bristles);
or long, fine, whip-like filaments, which are
moveable in all directions; or stiff styles or
pegs, which do not vibrate, and which answer
all the purposes of legs and feet.. The
Trichoda lynceus runs about, climbs, stands still,
or swims away, as nimbly as the most agile
quadruped, entirely by means of its
transparent pins. Even in these last-mentioned
members, no distinct muscular structure, nor
joints can be detected; they appear to be
mere prolongations and offshoots of the
sarcodic body and its shield or pellicle. It is
impossible, therefore, to imagine organised
life existing under conditions of greater
simplicity. Nature has economised both her
materials and her power, by deciding that
water shall be the habitat of her minims.
Indeed, it is difficult to conceive by what
means at all, any creature of the size of a
small Infusory could be made to fly through
the air, or to live on really dry land. But, as
we note with reverence and admiration,
nature has adapted her handiwork, in the
case of the Infusoria, to the physical conditions
required for the production of miniature
organisms of the extremest exiguity.

Those who doubt the simplicity of their
organisation, will find a proof of it in watching
the curious phenomenon of the
decomposition of sundry of them (not all), by what
is called diffluence, which the writer of this
has often and often witnessed. The creature
is alive, in high health and spirits, when, in
consequence of some cause, accidental or
intentional (such as the contact of air, or the
introduction of a little salt water, ammonia,
or spirit), it suddenly falls to pieces, breaks
itself up into particles, goes off like a column
of smoke, pours itself out, and so gradually
is reduced to atoms. You have before you
the singular spectacle of the decomposition
of a living creature piece by piece. Sometimes,
these detached fragments will swim away
separately for a short time and distance,
either each urged by its own individual
bristle which hangs to it, or by the bristles
remaining on the still-undissolved portion of
the body, until nothing is left. Sometimes
the Infusory, when it has partly destroyed
itself by diffluence, will check the progress,
and proceed on its course, half itself, as if
nothing had happened. The accomplished
performance of this whimsical freak has given
rise to many so-called species, which, in fact,
are nothing but self-mutilated animalcules.
Ehrenberg mistook dissolution by diffluence
for an act of reproduction, a laying of eggs
or spawn. He beheld the fact, but
misinterpreted it. Now, it is certain, that if the
Infusories were possessed of muscular fibre,
of a tegument capable of offering resistance
outside, and of a true stomach and intestines
inside their body, some indication of the
presence of those organs would be given
during the gradual dissection of the
creature's frame, which is effected by the
progressive decomposition of diffluence.

The order Infusoria, as at present limited,
is far from containing all the microscopic
animated beings that live in water. The
monads, certainly, are many of them merely
locomotive plants, such as volvoxes and the
zoospores of water-weeds; while others, most
probably, are the immature forms of larger
animalcules. Even the vibrios, including the
walking-stick worm, are now remanded to
the realms of botany; so also of the
Desmideæ and the Diatomaceæ, which were
regarded by Ehrenberg as infusory animalcules.
The water-fleas and other cyclopes,
which greatly resemble minute shrimps and
lobsters, belong to a separate division,
Entomostraca, of the class Crustacea. The little
eels or anguillules, of different species, found
in vinegar, paste, wet moss, and diseased
corn, are marched off by modern zoologists
to the class Entozoa, or intestinal worms.
The wheel-animalcules have been deservedly
promoted to a much higher rank than
Infusoria in the scale of being. Under the title