The fossil Echinidæ first appeared in the lower Ludlow limestone, and attained their maximum in the Cretaceous strata. A species of Diadema, with annulated hollow spines, common in the Chalk, still exists. Numerous species of the genus Clypeaster, remarkable for their flattened form, and known as lake urchins, are peculiar to the Tertiary strata and existing seas; and, lastly, five species of Spatangidæ, heart-shaped urchins, which lived in the Tertiary periods, still exist. In consequence of the porous texture of the solid calcareous parts of the Echinidæ, their fossil remains are commonly impregnated with pyrites or silex, without altering their organic structure, so that they exhibit a fracture like that of calcareous spar. _Echinodermata Holothuroïdea._ The Holothuridæ, or Sea-Cucumbers, are of a higher organization than the preceding Echinoderms. They are soft, worm-shaped, five-sided animals, covered by a flexible, leathery integument or skin, in which are imbedded a vast multitude of microscopic calcareous plates of reticulated structure. The mouth, which is placed at one end of the animal, is surrounded by ten bony plates forming a lantern, analogous to that of the Echinus; they support branching, tubular, and retractile tentacles, which encompass the mouth like a star. The tentacles are connected with sacs at their bases, and are extended and retracted by the injection of a watery liquid contained in them. Innumerable tubular, suctorial feet, precisely similar to those of the Echinus, are protruded and retracted through corresponding pores in the skin of the animal by a watery liquid, in sacs, at their bases. The water is supplied by a system of canals connected with an annular reservoir round the top of the gullet, which is supplied with water by a bottle-shaped bag at the mouth. Besides transverse muscles, five pairs of muscles attached to the lantern at the mouth, extend throughout the whole length of the animal. Nerve-chords from the ring at the gullet accompany these, and such is the irritability of this muscular system, that the Holothuriæ eject their viscera when alarmed or caught; but they have the power of reproducing them: sometimes they divide their whole body into parts. The respiratory organs are two very long and beautifully arborescent tubes veined with capillary bloodvessels. The circulation of the blood is similar to that of the star-fishes, but more complicated. The minute calcareous particles scattered independently in the tough leathery skin of the Holothuridæ remain as fine dust when the flesh is dissolved and washed away; but, upon microscopic observation, Mr. Gosse found that the forms of these particles are remarkable for elegance, regularity, and variety of structure, but that the normal form is an ellipse of open work built up of five pieces of a highly refractive, transparent, glassy material, having the shape of dumb-bells.