[Illustration: LARVA OF THE COMMON GNAT.
A. The body and head of the larva (magnified).
B. The respiratory apparatus, situated in the tail.
C. Natural size.]

To an intelligent person, who has previously obtained a general idea of
the nature of the Objects about to be submitted to his inspection, a
group of living animalcules, seen under a powerful microscope for the
first time, presents a scene of extraordinary interest, and never fails
to call forth an expression of amazement and admiration. This statement
admits of an easy illustration: for example, from some water containing
aquatic plants, collected from a pond on Clapham Common, I select a
small twig, to which are attached a few delicate flakes, apparently of
slime or jelly; some minute fibres, standing erect here and there on the
twig, are also dimly visible to the naked eye. This twig, with a drop or
two of the water, we will put between two thin plates of glass, and
place under the field of view of a microscope, having lenses that
magnify the image of an object 200 times in linear dimensions.

Upon looking through the instrument, we find the fluid swarming with
animals of various shapes and magnitudes. Some are darting through the
water with great rapidity, while others are pursuing and devouring
creatures more infinitesimal than themselves. Many are attached to the
twig by long delicate threads, several have their bodies inclosed in a
transparent tube, from one end of which the animal partly protrudes and
then recedes, while others are covered by an elegant shell or case. The
minutest kinds, many of which are so small that millions might be
contained in a single drop of water, appear like mere animated globules,
free, single, and of various colours, sporting about in every direction.