Page:Proceedings of the Royal Society of London Vol 60.djvu/55

Rh of the larva, and its nucleus is usually, b u t not always, elongated in th e same direction, so as to have a rod-like form. The whole structure, with pigm ent, lens-like body, and central granular cells, gives strongly the impression of a prim itive, light-perceiving organ. The pigm ent itself is lodged in the inner ends of th e ciliated and in te rmediate cells, and is, no doubt, the same pigm ent as th at observed by Metschnikoff* and S chulzet in the inner ends of th e ciliated cells in the larva of Sycandra raplianus. As the interm ediate cells pass into th e condition of granular cells, they leave the pigm ent behind, so th at the pigm ent is thickest in the region of the interm ediate cells, at the sides of the lens-like body. The larva is thus composed of four kinds of cells, which may be term ed the ciliated, interm ediate, granular, and central cells. Since the interm ediate cells are m erely a transitional form betw een the ciliated cells proper and th e granular cells, we have to reckon w ith three classes of cells only in the fully developed larva.

The fixation takes place by th e anterior pole of the larva, and th e granular cells grow round the ciliated cells. The m etam orphosis is complete in a few hours. Sections of fixed stages of the first day of fixation (fig. 4) show them to be composed of two very distinct cell Postlarval Development o f Leucosolenia variabilis, II. sp. 47

Fig. 4.—Section of larva shortly after fixation, the metamorphosis not quite complete. layers: (1) a compact central mass of cells, easily recognisable, by their opaque, irregularly shaped nuclei and vacuolated cell protoplasm, as the former ciliated cells, surrounded by (2) a single layer of flattened epithelial cells, the form er granular cells of the larva. No trace is to be found of the central cells, which appear to be throw n out, together with the pigment, at the metamorphosis. The inner mass is the future gastral layer of the sponge, the outer epithelium the future dermal layer.

‘ * “ Zur Entwicklungsgeschichte der Kalkschwamme,” ‘Zeitschr. f. Wiss. Zool.,’ vol. 24, pp. 1—14, Taf. I. t “ Ueber den Bau und Entwicklung von Sycandra ib., vol. 25, suppl., pp. 247—280, Taf. XVIII—XXI.