-work, sand, and
plants must be washed over several times, so that no injurious
substances may be introduced.

Ulva, or sea-lettuce, is to be found in abundance in all our small bays
and inlets at low tide. For the aquarium, those specimens which are
thick in texture, and of a dark green color, only are fit for
manufacturing air. Never be tempted to make use of the light green and
thin specimens, as they are not sufficiently matured, and will soon
decay if placed in the tank.

[Illustration: SCALLOPS.]

Scallops when young have a curious way of changing their location by
means of opening their shells and then closing them with great force,
which sends them off at an angle, and so they go dancing along the
bottom till they reach a spot that suits them. This shell-fish forms a
beautiful addition to an aquarium.

[Illustration: HERMIT-CRAB WITH SHELL.]

The silver-shrimp, with figured back (all other varieties must be
avoided), I have always considered as constituting a Board of Health in
an aquarium; for no sooner does the water become unhealthy than these
transparent and grasshopper-like creatures will make desperate attempts
to jump out of the tank. These shrimps, and the little hermit-crab, and
the buccinum (a small black sea-snail) are Nature's house-cleaners. They
are always on the look-out for decaying animal or vegetable matter,
which, if not in too large quantities, they speedily devour.

I have seen these black snails gather on a dead fish from a distance of
half a mile; in less than a day's time nothing was left of the fish but
his bones and scales, and these were picked so clean that they had a
polished look. These snails are provided with ribbon-like tongues, from
which project a great number of minute and beautifully constructed
teeth. By passing these tongues backward and forward rapidly they cut
their food down much as a mowing-machine cuts grass. These snails are
the scavengers of all dead fish and vegetable substances found in our
bays and rivers, and to them we owe a great deal of the purity of our
waters.

The little hermit-crab lives alone in an empty shell, which he carries
about with him wherever he goes. His reason for living in a shell is
because the hind part of his body is soft, and not protected with a hard
shell, like the fore part of his body. The end of the soft body of the
hermit-crab is provided with hooks, or claspers, with which he holds on
to the inner chamber of his shell so ti