V. Skeletal development in Arbacia , Echinarachnius and Leptasterias

The echinopluteus of the genusArbacia has been known since 1853. Echinoplutei of the species at present under investigation were first reared in 1880 by Fewkes, and two years later Garman and Colton (1882) succeeded in rearing them through the metamorphosis. The Mediterranean speciesArbacia lixula , L. (syn.A .pustulosa , Gray) has also been reared through metamorphosis, and Übisch (1913) was the first to attempt an analysis of the test of the imago. The composition of the corona in the imago ofA .pustulosa , as described in the paper just referred to (Übisch, 1913), is very different from that in the imago ofe. g .Echinus orStrongylocentrotus . The opportunity of working at the Marine Biological Laboratory, Woods Hole, presenting itself, it was thought that a study of the development of the test inArbacia punctulata , Gray, might reveal some points of interest. Accordingly, cultures of this common sea-urchin were started on July 28, 1926, and the echinoplutei were fed on the diatomNitzschia closterium W. Sm. formaminutissima . Forty days later (September 6th) the first imago was obtained and the echinoplutei continued to metamorphose throughout the rest of September. Early in August a shallow glass vessel containing filtered sea-water was infected with plankton obtained by towing, and, by the first week of September, the bottom and sides of the vessel were well coated with diatoms. Many of the imagines, which measure 0·5 mm. in diameter including the spines, were transferred to this vessel and a number increased considerably in size. The largest specimen obtained in this way was 1·63 mm. in diameter inclusive of the spines; the diameter of the test alone was 0·9 mm.