In the pelagic plankton, single-celled animals of the groups called foraminifers and radiolarians are exceedingly prominent, and play an enormous part in the economy of the sea, although almost or quite microscopic in size. They are incased in chambered shells of lime or flint; and over vast areas in warm latitudes the ocean floor is so thickly covered with the dead shells of one kind that the mud is called globigerina ooze. They are the eaters of the microscopic plants, and themselves are food for a wide variety of hydroids and jellyfish, large and small, whose silvery forms are often visible to the voyager, and which are mostly responsible for the pale stars of phosphorescence that shine about his prow and glorify his wake in dark nights. The queen of these far swimmers is the radiant Portuguese man-of-war. In the night a dragging fine-meshed net will capture more than by day of the plankton, because many little creatures that in daylight sink to considerable depths come to the surface at night. Rising a step to the worms, we find them comparatively rare, but one kind of marine flatworm that abounds in midocean is rose-red and several inches long. Much more numerous is another flatworm, Sagitta, "which along with copepoda, salpæ, pteropoda and radiolaria, everywhere constitute the bulk of the small pelagic organisms" captured by towing nets. Like almost all of these usually defenseless creatures they are perfectly transparent, but some of them depart from the rule of pale blue in tint and shine in bright red. A longer step takes us to the Crustacea, represented in the pelagic plankton by queer little shrimplike forms that in countless hosts of individuals play a part in the ocean comparable to that of insects on land. The copepods are the most numerous probably--little things only a fraction of an inch in length, but amazingly abundant, and the principal users of plant food. Their relatives, the little ostracods, have similar habits, and are noted for their intense phosphorescence. Haeckel relates that on his way to Ceylon he saw the entire sea like a twinkling ocean of light, and his microscope showed him that it was made by throngs of ostracods, with some jellyfishes, salpæ and worms. Crustaceans of higher rank abound also. In northern waters species of Schizopoda, small, transparent prawns with red spots around the mouth and big, black eyes, swarm in enormous numbers, and are known to the fishermen as "kril."