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 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 can be counted. As these are all double, this would give five thousand seven hundred and sixty cells to the square inch. Now a moderate-sized specimen will afford, with all its convolutions, at least one hundred square inches of wall, which would contain a population of five hundred and seventy-six thousand inhabitants,--a very large city. So says Mr. Gosse. We cannot forbear, with him, from quoting Montgomery's lines on the labors of the coral-worms, which modern science has enabled us to study in our parlors.