Fig. 68.—Grantia compressa
Fig. 69.—Spicules of Grantia, magnified
Small portions of the skeleton, examined under the microscope, will show the nature of the calcareous spicules of which it is composed. These consist of minute needles and stars, the latter having generally either three or four rays.
We give figures of three of the calcareous sponges of our shores, the first of which (Grantia compressa) resembles little oval, flattened bags, which hang pendulous from rocks and weeds, sometimes solitary, but often in clusters. The smaller openings are thickly scattered over the flat sides of the bag, and the larger ones, through which the water is expelled, around the margin. When the sponge is out of the water and inactive, the two opposite sides of the bag are practically in contact, but, when active, the cavity is filled with water by means of the whip-cells that line it, and the sides of the sponge are then more or less convex.
