17
examination. If we put a few particles of the polythoa, or some fragments gently torn from the inner surface of the sponge, into strong nitric acid for a day or two, the animal and extraneous matter will dissolve away, and the silicious particles, including the spicules, will alone remain. These must be well washed in many waters to remove all traces of the acid, and will then be ready for examination with the microscope, or for mounting in Canada balsam, if it is desired to keep them as permanent objects; taking care in the latter case, not to break the more delicate forms by too much pressure on the covering glass. Proceeding thus with the polythoa, we find that the spicules have a great tendency to assume the cruciate form, the most common of all being described in scientific terminology as cylinuro-cruciform with densely spiculete shafts. In some other forms, the shafts of the cross are spiculate only at the extremities, and are much more delicate in contour. We meet with others in which the lateral shafts of the cross are reduced to mere rudimentary projections, whilst still in others they are absent altogether and the spicule is a short thick cylinder studded all over with sharp spines. Of these straight or nearly straight spicules we find several kinds, some spinous only at the ends, others with a small central enlargement, and lastly we meet with delicate spicules, some in the form of crosses presenting perfectly smooth outlines. In the sponge, some of the commonest, although not the most striking, forms, are these spiculated cruciform spicules, with a little foot-piece in the form of a cross from which springs a much spiculate stem. Dr. Wright tells us that these are chiefly found round the oscula or openings of the aquiferous channels and lining the cavities of the sponge, attached by the little foot piece to the sarcode mesh. He says,—to quote his own words—“From the peculiar way in which they are placed on the edges of the meshes, and from the fact that the barbs on the stem of the spicules all point in one direction, it is possible that while it would be easy to glide over the slimy sarcode down into an osculum, return would be no easy task, as
