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.
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