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.

"Millions on millions thus, from age to age,
With simplest skill, and toil unweariable,
No moment and no movement unimproved,
Laid line on line, on terrace terrace spread,
To swell the heightening, brightening, gradual mound,
By marvellous structure climbing towards the day.