protected behind by Nature, the homeless crab wanders about seeking a
lodging. Presently he meets with an empty shell, and, after probing it
carefully with his claw to be sure it is not tenanted, he pops into it
back foremost in a twinkling, and settles himself in his new house.
Often, too, he may be seen balancing the conveniences of the one he is
in and of another vacant lodging he has found in his travels; and he
even ventures out of his own, and into the other, and back again, before
being satisfied as to their respective merits. In all these manoeuvres,
as well as in his daily battles with his brethren, he is one of the
drollest of creatures.

As we advance in our practice with the aquarium we may venture to
introduce more delicate lodgers. Such are the beautiful family of the
_Annelidae_: the _Serpula_, in his dirty house; and the _Terebella_,
most ancient of masons, who lays the walls of his home in water-proof
cement.

The great class of Zoöphytes can be introduced, but many varieties of
them will be found already within the aquarium, in the company of their
more bulky neighbors. These peculiar creatures, or things, form the
boundary where the last gleam of animal life is so feeble and flickering
as to render it doubtful whether they belong to the animal or vegetable
kingdom. Agassiz calls them _Protozoa_,--Primary Existences. Some divide
them into two great classes, namely: the _Anthozoa_, or Flower-Life; and
the _Polyzoa_, or Many-Life, in which the individuals are associated in
numbers. They are mostly inhabitants of the water; all are destitute of
joints, nerves, lungs, and proper blood-vessels; but they all possess
an _irritable_ system, in obedience to which they expand or contract at
will. Among the _Anthozoa_ are the Anemones; among the _Polyzoa_,
are the Madrepores, or Coral-Builders, and many others. Many are