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Show 1873.] DR. J. S. BOWERBANK ON CEYLONESE SPONGES. 31 Its diameter very slightly exceeds that of one of the large spicula of the sponge ; and its figure, full'of contortions, is just such as we m ay imagine that of a dying, struggling, slender worm would have been under such circumstances. It is so far covered and partially hidden by the surrounding sarcode, and so completely emptied of its former contents, that nothing but its form is distinctly visible ; but be it what it m a y it is certainly no part of the organization of the sponge. It is no uncommon event to find in living sponges such slender worms attempting to prey either on the dermal surface or on the interstitial membranes within the sponge. With such a structure as that of the specimen in course of description, a common investing dermal membrane could scarcely be supposed to be required or to exist, and the true dermis must be sought at the external surface of each of the skeleton-fistulse; but as these organs are so small and their parietes so thin, it is extremely difficult to discriminate the dermal tissue from the parietes of the fistulse. In some of the specimens mounted in Canada balsam, their surfaces are frequently much obscured by sarcode; but in some parts there are strong indications of a very delicate dermal membrane enveloping the fistula. Some of the large flecto-acuate spicula are procumbent and closely adherent for the whole of their length to the surface of the fistulse. They are disposed in lines more or less in accordance with its long axis ; and in this position they perform the office of skeleton-spicula, strengthening and supporting the delicately constructed fistula by their longitudinal position on its surface, and maintaining it in its normal position; while others of them are projected as defensive organs from its surface. Very few of the small incipiently spinous defensive spicula are seen to be procumbent; and by far the greater number are projected backward at various angles to the surface of the fistula. The direction in which both descriptions of defensive spicula are projected from the surfaces of the fistulse is more frequently backward than forward, or at right angles to the surface. This seems to indicate that they are carnivorous feeders, and that this position of the spicula is destined to obstruct the egress of any small annelids that m a y have entered the body of the sponge, and to destroy them for its own nutrition. N o adventitious substances are incorporated with the skeleton of this species as in the other two known ones, and all its spicula are undoubtedly secreted by itself; and in every respect it forms the best type of the genus Haliphysema with which we are acquainted. DESCRIPTION OF THE PLATES. PLATE V. Spongionella Holdsworthii. V\". 1. The type specimen, natural size, exhibiting the numerous oscula within the cup or upper surface of the sponge. |