LUMINOUS ECHINODERMS.
In the fourth grand branch of the animal kingdom, numerous creations
are known which exhibit luminosity. The Echinoderms, as they are
termed, are not well known to those who are not familiar with the
seashore. To those who visit the marine beaches, one of the first
objects that is met cast up by the tide, either fresh from its
ocean bed among the rocks, or lying cast up high and dry amongst
the vast masses of kelp, algæ, and other marine _débris_, is
a sea-urchin,--so called for want of a better name, although the
spines with which it is powerfully armed give good color to the
nomenclature. The term Echinoderm is used to express all the kinds,
as they have spines on the skin. As the arrangement of this division
of Nature suggests, the creatures which are embraced here are next
farther advanced in perfection of structure from the third, which
includes the corals and sea-anemones. The animals are of most varied
shape, exteriorly most unlike each other, yet internally possessing a
structure each characteristic of the type. The sea-stars, forms quite
as common as the sea-urchins which we first mentioned, are closely
alike in structure, though so different in shape. Yet another form is
seen in the celebrated trepang, which is dried, smoked, and sold to the
Chinese for food,--a great luxury to them. Small species are found on
our coast.
In some of these creatures the luminous property has been
observed,--which usually surrounds the entire animal,--a pale light,
rendering the object a beautiful one against the dark background of
the ocean bottom. It is needless to say that the human eye has not
penetrated these vast depths; but the ingenuity of the scientist has
resulted in the invention of means by which the smallest as well as the
largest of these strange creatures are dragged from their deep abode.
Echinoderms are extremely numerous; on the Florida reefs we have often
found it impossible to wade through considerable areas, where a kind of
sea-urchin having long, slender black spines was so numerous as to pave
the entire sea-bottom, and in certain localities in Long Island Sound
we have seen the bottom fairly carpeted with star-fishes. It is not
surprising, then, that the dredges of the “Challenger,” “Porcupine,”
“Talisman,” and other ships fitted out for scientific investigation,
often came up loaded to overflowing with star-fishes, showing that the
deep sea is equally populous with these living stars.
These deep-sea forms, especially of the genera _Asterias_ and
_Ophiura_,[25] are remarkable for their brilliancy, even when
taken from their native element. When the bottom off the coast of
Ireland was dredged by the “Challenger,” an extraordinary number of
luminous star-fishes were brought up from a depth of two-thirds of a
mile. Several specimens are most noticeable for their brilliancy;[26]
they appear as if burning internally with heat of great intensity.
Even the mud about them was bespangled with luminous specks; and Sir
Wyville Thompson says that in many instances every thing brought up
in these waters was luminous. The light of one of the star-fishes
was a brilliant green, and seemed to spring from the centre of the
disk; flashing out now upon one arm, again upon another, or suddenly
illumining the entire star in a brilliant aureola of phosphorescence.
This resplendent creature is especially common, according to Sir
Wyville Thompson, off the coast of Stornaway and Shetland; and the
nets, when hauled in, were often overladen with masses of these
gorgeous forms, which emitted a light of brilliant uranium green.
Curiously enough, the young star-fishes exceeded the adults in the
richness of their display. The gleams were not constant, but extremely
erratic, appearing and re-appearing in a bewildering manner; and,
according to the same naturalist, the most striking exhibitions were
seen in very young ones.
The star-fishes known as Ophiuroids are among the most abundant of
deep-sea forms. On the “Challenger,” about several hundred species were
brought up in the trawl from a depth of from half a mile to two and a
half miles. In our own waters, two kinds[27] have been observed to emit
a light of singular brilliancy.
Even more beautiful than these, as regards their luminosity, are the
Brisingas,[28] one of which is shown with its light in Plate VIII.,
Fig. 1. This animal has nineteen long, snake-like arms, branching from
a small central circular body. Its color in the daylight is a rich
orange red; but at night, when taken from the dredge, it displays a
vivid phosphorescence.
This attractive animal was first observed near Bergen, Norway, by
Charles Abjördsen, who took a specimen in two hundred fathoms of water.
Regarding it, he said, “it is a true _gloria maris_,” and gave it the
name of _Brisinga_, one of the jewels of the Goddess Freya.
The Brisingas have the faculty, common to many of their allies, of
casting their arms when touched; so that it is extremely difficult to
take them intact. In lifting an _Astrophyton_[29] from a branch of
coral, we have had it drop into myriads of pieces; so that there was a
mimic rain of arms upon the bottom. This we found could be avoided by
making the transfer under water, and, when the “basket-fish” was safely
in the jar, killing it by the introduction of alcohol.
As to the cause of the light in the star-fishes, little is known.
Quatrefages, after a careful examination of an Ophiuran, came to the
conclusion that the light emitted was due to muscular contraction;
observing it arising between the plates of the arms and not on the
disk, where, however, it has been seen since his observations were
made. Professor P. Martin Duncan found upon examining a specimen,
brought from the icy sea of North Smith’s Sound, by Sir George Nares’s
expedition, that it had a delicate mucous envelope, which, he thought,
in the young covered the plates and bases of the spines. In this filmy
covering, he suggests, may be found the seat of the illuminating power.