The hollow, five-sided, calcareous, jointed stem of the living Pentacrinite is filled with a spongy substance, and supports a cup on its summit, containing the digestive organs, mouth, and tentacles of the animal. The cup is formed of a series of calcareous plates, and from its margin five long many-jointed rays diverge, each of which is divided into two-jointed branches. Groups of curled filaments, called cirri, are placed at regular distances from the bottom of the stem to the extremity of the rays, while, on the opposite side of the rays, there are groups of feathery objects called pinnæ at each joint. Food is caught by the tentacles and digested by the stomach and viscera at the bottom of the cup, from whence vessels diverge through a system of canals in the axes of the rays, pinnæ, and down the stem, all of which convey sea-water mixed with nutritious liquid, for the nourishment of the animal. The genus Comatula are star-fishes, believed to have alternately a fixed and a free state. Mr. J. V. Thomson discovered that the Pentacrinus Europæus is merely the fixed state of a Comatula. These star-fishes have pairs of pinnæ placed at regular distances along their long-jointed rays, and in the pinnæ sacs containing eggs are placed as far as the fifteenth or twentieth pair. The eggs yield active ciliated larvæ, which attach themselves in the form of flat oval disks to corallines and sea-weeds. By degrees they develop a stem, about three-fourths of an inch high, with twenty-four distinct joints. Its expanded top bears five sulphur-coloured bifurcating rays with their pinnæ and dorsal cirri. A mouth is formed in the centre with its tentacles, and a lateral prominent vent. The actual change of a Pentacrine into a Comatula has not been seen, but as the small Pentacrinites disappear in September, at which season the Comatulæ appear, it is believed that when full grown the top of the fixed Pentacrinite falls off and becomes a Comatula, which swims backwards with great activity by striking the water alternately with its long rays. The Pentacrinus caput-Medusæ, which is fixed by its stem to sea-weeds and zoophytes, forms a most beautiful object for the lower magnifying powers when viewed in a fluid by a strong refracting light. _Echinodermata Echinoïdea._