OCR Text |
Show 608 DR. J. S. BOWERBANK ON ALCYONCELLUM [Nov. 16, which they are immersed. Their complete immersion in the sarcode immediately beneath the dermal membrane indicates in a very satisfactory manner their peculiar office as expansile portions of the dermal system, and as organs of defence for the preservation of that important portion of the structure of the sponge. The specimen of Hyalonema mirabile is in a much finer state of preservation than any of those which I formerly examined, and described in the 'Proceedings' of this Society for 1867; and it has enabled m e to complete the description of many parts of this extraordinary species of sponge, which the dilapitated condition of the specimens to which I then had access prevented me from determining with accuracy. The total height of the specimen received from the ** Challenger' is sixteen and a half inches. The basal spongeous portion is of a conical form, two inches in height; and the average diameter of the base of the cone is two and a half inches. It is a fortunate circumstance that the basal membrane and that of the upper or conical portion of tbe spongeous mass are both in a perfect state of preservation. The basal portion of attachment is very sinuous and irregular, as if it had been adherent to an undulating surface which had been of somewhat soft consistence, so that it had been separated from it without the destruction of the basal membrane of the sponge. The apex of the spongeous mass closely embraces the spiral column of the cloacal system ; and the lower part of the spiral column is completely buried in the basal sponge. The corium, in a more or less perfect state, extends from the apex of the spongeous cone for about seven inches of the remaining portion of the spiral column of spicula ; the remainder of which is bare, but spirally twisted to very near its distal termination. The corium is studded with the usual mamilloid oscular organs; and none of them exhibited the slightest indication of polypiferous contents, which we should naturally have expected to find, had such parasitical creatures been present, in a living specimen fresh from the sea, as the one iu course of description evidently was. The adherent basal surface of the sponge has a distinctly marked boundary-line, consisting of a well-produced slightly compressed projecting ridge of membrane; and a difference of the structure of the basal and the upper portion of the dermal membrane is distinctly visible to the unassisted eye. This difference in their aspects arises from modifications in the reticular structures of the two parts to fit them for their respective offices. When small portions of each of the parts of the membrane were mounted in Canada balsam, their differences in structure became strikingly evident. The same forms of spicula were to a greater or a less extent present in both, but the modes of their distribution and arrangement were very different. The rete of the basal portion is comparatively compact and strong, and closely approaching regularity. It is constructed- with the long infiato-acerate spicula which are common to both parts of the membranous skeleton structure, but with a considerable mixture of large rectangulate sexradiate spicula, which |