The
habit of keeping marine animals in tanks is happily growing constantly
more popular, and before long the beauty of these inhabitants of the
ocean will be as familiar to us as that of Birds and Insects. Many of
the most beautiful among them are, however, difficult to obtain, and
not easily kept alive in confinement, so that they are not often seen
in aquariums.
Having thus endeavored to sketch each different kind of Echinoderm, let
us try to forget them all in their individuality, and think only of the
structural formula that applies equally to each. In all, the body has
three distinct regions, the oral, the ab-oral, and the sides; but by
giving a predominance to one or other of these regions, a variety of
outlines characteristic of the different groups is produced. In all,
the parts radiate from the oral opening, and join in the ab-oral
region. In all, this radiation is accompanied by rows of suckers
following the line of the diverging rays. It is always the same
structure, but, endowed with the freedom of life, it is never
monotonous, notwithstanding its absolute permanence. In short, drop
off the stem of the Crinoid, and depress its calyx to form a flat disk,
and we have an Ophiuran; expand that disk, and let it merge gradually
in the arms, and we have a Star-Fish; draw up the rays of the
Star-Fish, and unite them at the tips so as to form a spherical
outline, and we have a Sea-Urchin; stretch out the Sea-Urchin to form
a cylinder, and we have a Holothurian.
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