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Show 128 DR. J. E. GRAY ON PLACOSPONGIA. [Jan. 24, axis covered externally with separate laminae; the axis and laminae composed of closely adherent siliceous globules with a granular surface, and separated from each other by a layer of sarcode armed with siliceous spicules. The genus may be thus described :- PLACOSPONGIA. The sponge hard, angular, stony, angularly branched. The axis solid, formed of closely packed siliceous globules with an areolated tubercular surface, and covered with variously shaped hard plates of similar tubercular siliceous globules, having an areolated appearance on the surface under the microscope. The outer plates differ greatly in size and form ; but they meet at the edges, and rarely one edge slightly overlaps the other, giving the sponge an angular appearance. The axis is separated from the superficial plates by a continuous layer of sarcode furnished with bundles of nearly parallel pin-shaped spicules, which form columns diverging at right angles from the outer surface of the axis to the inner surface of the outer plates. The external plates are increased in size by the addition of new matter on the circumference, leaving indistinct concentric lines of growth on the outer surface. It is the manner of growth that makes them look so like the fronds of a large Melobesia. Fig. 1. Placospongia melobesioides, Gfray. 2. Cross fracture, showing the axis, sarcode, and outer lamina;. 3. Siliceous globule. 4. Pin-shaped spicule of sarcode. PLACOSPONGIA MELOBESIOIDES. Var. 1. Sponge thick, with short angular branches, chalky white. Hab. Borneo, B.M. |