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 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.
"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.
Each wrought alone, yet all together wrought,
Unconscious, not unworthy instruments,
By which a hand invisible was rearing
A new creation in the secret deep.
…..I saw the living pile ascend,
The mausoleum of its architects,
Still dying upwards as their labors closed;
Slime the material, but the slime was turned
To adamant by their petrific touch:
Frail were their frames, ephemeral their lives,
Their masonry imperishable."
The deep-sea soundings taken recently for the Atlantic telegraph have demonstrated the existence of organic life even at the bottom of the ocean. Numerous living Infusoria have been brought to the light of day, from their hidden recesses, by the lead. "Deeper than ever plummet sounded" before these latter days, there exist myriads of minute creatures, and of Algae to furnish their food. It is an unanswered problem, How they can resist the enormous pressure to which they must be there subjected, amounting, not infrequently, to several tons to the square inch. And still another point of interest for us springs from this. It is an inquiry of practical importance to the aquarian naturalist, How far the diminished pressure which they meet with in the tank, on being transferred from their lower homes to the aquarium, may influence their viability. May not some of the numerous deaths in the marine tank be reasonably attributed to this lack of pressure?
What a difference, too, has Nature established, in the natural power to resist pressure, between those creatures which float near the surface and those which haunt the deeper sea! The Jelly-fish can live only near the top of the water, and, floating softly through a gentle medium, is yet crushed by a touch; while the Coral-builder bears the superincumbent weight of worlds on his vaulted cell with perfect impunity.
Another important question is, How far alteration in the amount of light may affect the more delicate creatures. What fishes do without light has been solved by the darkness of the Mammoth Cave, the tenants of whose black pools are eyeless, evidently because there is nothing to see. The more deeply located Infusoria and Mollusks must dwell in an endless twilight; for Humboldt has found, by experiment, that at a depth one hundred and ninety-two feet from the surface the amount of sunlight which can penetrate is equal only to one-half of the light of an ordinary candle one foot distant.
Thus ever in gloom, yet in a state of constant safety from storms and the agitations of the upper air, the thousand forms of low organic life and cryptogamic vegetation live and thrive in peace and quietness.