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Show 1867-] DR. J. S. BOWERBANK ON HYALONEMA MIRABILE. 29 form, and are of a complicated and very interesting structure. They consist of a short stout cruciform base, with a long spicular ray as-cendingly and entirely spinous, projected at right angles from the centre of the basal radii. The spines on the spicular ray are similar in form and mode of disposition to those of the external defensive spicula, but very much longer in proportion to the size of the spiculum, frequently exceeding in their length the diameter of the shaft on which they are based (Pl. V. fig. 6, and fig. 1 b in situ). The radii of the cruciform bases are also slightly spiculated towards their apices. They are thickly distributed on the fasciculi of the skeleton, and frequently equally so on one side of the interstitial membranes, probably that which forms the surfaces of the interstitial spaces, and they are especially abundant near the exterior of the sponge. The four basal radii appear firmly cemented to the membrane- but not immersed in its substance, as they do not appear to leave their impression when removed from it, nor do they bring any portion of the membrane away with them. In some part of the tissues these spicula are very much modified in form. In ordinary cases we find the basal radii short and stout, and not more than a fourth or a fifth of the length of the spicular ray; while in other cases the basal rays are very nearly as long as the spicular one, the only difference in their structure being that the latter is very strongly spinous, while the former have the spines comparatively very slightly produced. The interstitial membranes, when not covered with spiculated cruciform spicula, are often abundantly furnished with long slender flexuous acerate tension spicula, with a central inflation indicative of their being an incipient condition of either rectangulated hexradiate or rectangulated triradiate forms ; and the latter one occasionally is found among them (Pl. V. fig. 7). The interstitial spicula of this sponge are very numerous, and exceedingly various in size and form. They are of three very distinct descriptions :-first, rectangulated hexradiate, large and small (Pl. V. figs. 8, 9) ; second, fimbriated multihamate birotulate (Pl. V. figs. 2, 3 ) ; and third, cylindro-cruciform (Pl. V. figs. 10, 11, 12, 13, 14). ' The first of these forms abound immediately beneath the apparent line of the dermal membrane in the large basal mass of the sponge; the greater portion of them are large, and they are disposed with a considerable approach to regularity, and amongst them there are frequently groups of the smaller variety of this form (Pl. V. fig. 8). They are also rather abundant near the basal portion of the spiral column of the cloacal system of the sponge, and they are found more sparingly dispersed in all parts of the basal mass. Generally speaking the whole of the six radii are fully produced; but occasionally pentradiate forms are found. The second form or fimbriated multihamate birotulate spicula are generally found dispersed amid the interstitial tissues of the large basal mass of the sponge. There are usually not more than one or |
