CHAPTER IV.

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.