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Show 1873.] DR. J. S. BOWERBANK ON CEYLONESE SPONGES. 27 Holdsworth from the 8-fathom part of the Great Pearl-bank at Ceylon. It was immediately immersed in spirit; and m y friend states that it " has not appreciably altered in appearance, shape, or colour since I first took it in m y hand. It is the only one of the kind I have seen." The regular conical form of this species is very characteristic, as I do not know any other species of the genus, either British or foreign, that has any well-defined form. The surface characters also are unlike those of any other known species. The internal structure very closely resembles that of our British species D. fragilis, exhibiting precisely the same mode of increment that I have described in m y ' Monograph of British Sponges,' vol. i. pp. 78 & 211, figs. 270-272, as prevailing in that species. The skeleton- structure is more membranous than fibrous, the latter being frequently more like thickened membranous edges than true fibres, with the membrane extending between them. Most frequently the membranous extensions are completely covered by a single stratum of particles of sand firmly cemented to them, which are so closely packed as to completely resemble a piece of fine mosaic work ; and no artist could adjust the positions of the large and small pieces of sand with greater precision than that exhibited on the membranes of the sponge. There is something more than the mere adhesion by chance in the attachment of the grains of sand to the membrane. The close and accurate adjustment of the particles to each other, the filling in of all the angles as completely as the most careful workman in mosaics would have adjusted them to each other, plainly indicate something more than a mere dispersion of the grains over the membranous surface. W e find the fibres projected from the mass of the sponge in search of grains of sand with which to form the artificial skeleton of the animal; and it is but a step further in the organization possessed by the animal to imagine that this beautiful arrangement of the particles of sand on the membrane is achieved by the contractile power that we know those tissues to possess. It is well known that they can contract any portion of their own substance, and thus open pores for the imbibition of nutriment, and, if alarmed, again close them so completely that their very position becomes invisible ; and it is but a step further to believe that the same description of voluntary contractile power has enabled them so to operate by contractions of the tissue as to bring every molecule of sand cast upon its surface into close conjunction with each other in the complete and beautiful manner that obtains in this sponge, and thus form the exquisite mosaic arrangement that m a y be seen on its membranes. If we are to judge by the amazingly various and beautiful structures exhibited in the sponges, we must certainly credit them with an amount of instinctive power they have hitherto never been imagined to possess, and assign them a much higher position amidst the lower animals than they have hitherto been supposed to merit. Occasionally there are spots of interstitial membrane unocccupied by grains of sand ; and these were abundantly furnished with lenticular nucleated cells of rather unequal sizes, the nucleus being visible in the largest ones only. |
