Development and metamorphosis of the sea-star, Astropecten scoparius Valenciennes." 1. Astropecten scoparius develops to a bipinnaria with simple ciliary bands and short bipinnari arms through a wrinkled blastula by holoblastic, radial cleavage. 2. About seven days after insemination, the posterior portion of the bipinnaria becomes swollen and fine spicules appear on it, while the anterior portion, the stalk, remains unchanged. 3. Metamorphosis takes place gradually at the posterior portion, while the metamorphosing bipinnaria is pelagic. Two weeks after insemination, the stalk rapidly shrinks and the larva sinks to the bottom. 4. About 18 days after insemination, the juvenile completes metamorphois with the opening of the mouth. The newly metamorphosed juvenile is 600 mu in diameter and each arm bears two pairs of the tube-feet, each having a sucker at the tip, and one terminal tentacle with red eye-spot. 5. The aboral skeletal system of the juvenile immediately after metamorphosis is composed of one central, one madreporic, ten radial and interradial plates, in addition to five terminal plates on the arms. 6. The juveniles smaller than about one cm in R do not bear some of the diagnostic features in this species. 7. A characteristic feature of the development of Astropecten, i.e., the lack of a brachiolaria stage, is stressed. The term ""nonbrachiolarian type"" is tentatively proposed to distinguish the development of Astropecten and Luidia, which do not pass through a brachiolaria stage, from the usual indirect type of development."