All are familiar with the mode in which the Soldier or Hermit Crab takes possession of and lives in the shells of Whelks and Snails.  Poorly 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 Zooephytes 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 microscopic, and belong to the class of animalcules called Infusoria.

A very remarkable quality which the Infusoria possess—­one very useful for the aquarium, and one which would seem to settle their place in the vegetable kingdom—­is that they exhale oxygen like plants.  This has been proved by Liebig, who collected several jars of oxygen from tanks containing Infusoria only.

A piece of honeycomb coral (Eschara foliacea) is easily found, and, when well selected and placed in the aquarium, may continue to grow there by the labors of its living infusorial tenants:  they are not unworthy rivals of the Madrepores, or deep-sea coral-builders of warmer latitudes.  The walls of its cells are not more than one-thirtieth of an inch in thickness, and each cell has its occupant.  So closely are they packed, that in an area of one-eighth of an inch square the orifices of forty-five cells