Studies in Spicule Formation. : VII.—The Sclerohlastic Development of the Plate-and-Anchor Spicules of Synapta, and of the Wheel Spicules of the Auricularia Larva.

Abstract

(1) The spicule first appears as a granule contained in a syncytium, in which, however, the scleroblasts retain their individuality to some extent, which is not the case in the syncytium of Synapta. (2) The spicule becomes disc- and then cup-shaped, develops the spokes as outgrowths from the margin of the disc, and finally forms the felly of the adult wheel, the spicule during the whole of its development being enclosed by the syncytium in which all the nuclei (scleroblasts) are situated on its internal side, i. e. away from the body-wall against which the spicule lies. (3) The extension of the scleroplasm depositing the spicule is determined by the growth of the spicule itself, and is not the result of a mould-forming tendency on the part of the protoplasm, as Chun asserted. (4) The situation of the heavy wheel and globe spicules at one (the lower) extremity of the larva determines the position which the larva assumes in the water--the spicules weight the larva.