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.