Medusæ of different species are met with in every sea from the equator to the poles. They are eminently social, migrating in enormous shoals to great distances. The largest shoal of young sea nettles on record was met with in the Gulf Stream, off the coast of Florida, by a vessel bound for England. The captain likened them to acorns; they were so crowded as completely to cover the sea, giving it the appearance from a distance of a boundless meadow in the yellow leaf. He was five or six days in sailing through them, and in about sixty days afterwards, on returning from England, he fell in with the same school, as the sailors call it, off the Western Islands, and was three or four days in sailing through them again. Mr. Piazzi Smyth, when on a voyage to Teneriffe in 1856, fell in with a vast shoal of medusæ. With a microscope he found part of the stomach of one of these creatures so full of diatoms of various forms—stars, crosses, semicircles, embossed circles and spirals—that he computed the whole stomach could not have contained less than 700,000. The flinty shells of the diatoms ejected in myriads by the medusæ, accumulate in the course of ages into siliceous strata, which, heaved up by subterranean fires, at length become the abode of man. Thus gelatinous transparent beings indirectly aid in forming the solid crust of the earth by means of the microscopic vegetation of the sea. _Ciliograde Hydrozoa._ The ciliograde Acalephæ, which form four orders and many genera, and which swim by means of symmetrical rows of long cilia, are represented on the British coasts by the Cydippe pileus and the Beroë Forskalia (fig. 116), little delicately tinted, gelatinous, and transparent animals that shine in the dark. The Cydippe pileus is a globe three-eighths of an inch in diameter, like the purest crystal, with eight bands of large cilia, stretching at regular distances from pole to pole. A mouth, surrounded by extremely sensitive tentacles, is situated at one pole, the vent at the other. The Cydippes poise and fix themselves to objects by means of two very long tentacles, fringed on one edge by cirri, that is, short curled tentacles. These cirrated tentacles, which in some species stretch out to more than twenty times the length of the animal, can be instantaneously retracted into cavities at the posterior end of the body, while, at the same time, the marginal filaments are as rapidly coiled up in a series of close spirals. The whole of these complex organs are enclosed within the limits of a pin’s head. [Illustration: Fig. 116. A, Cydippe pileus; B, Beroë Forskalia.]