OCR Text |
Show 20 DU. J. S. BOWERBANK ON THE SPONG1AD.K. [Ja'.l. /, fibres, forming a rather regular elongate reticulation ; and the same mode of fibrous arrangement prevails on the bulbous surface of the sponge, but not with quite so much regularity in its structure. Beneath the fibrous stratum at the dermal surface, the sponge is principally composed of a mass of interstitial reticulate skeleton-structure, with a few long threads of spiculo-fibrous tissue running amidst it. The reticulate interstitial skeleton-structure is uniform and regular in its construction, the rete being unispiculous, and the areas rarely exceeding the breadth of the length of one spiculum ; the spicula are of the same size and form in all parts of the sponge. There is a considerable amount of similarity in form and structure between this Australian species and our British one, Desmacidon Jeffreysii. Both are bulbous in form, and they are both furnished with long fistulous cloaca*:; but those of D. fistulosa are very much more delicate in their structure than the similar organs in D. Jeffreysii ; and in the latter species there are large root-like basal processes which serve to elevate the sponge above the mass to which it may have been attached; but no such organs appear to have existed in D. fstulosa. The spicula in both sponges are exactly alike in size and form, but they do not agree in their mode of arrangement. In the dermal membrane of D. Jeffreysii they appear always to assume more or less of a reticulate arrangement; but in the like organ of D. fistulosa they are irregularly dispersed. The interstitial ha-lichondroid part of the skeleton in both species also differs. In D. fstulosa it is very delicate and the rete is unispiculous, while in D. Jeffreysii the rete is often constructed of two or three spicula, and the areas are much larger and more irregular in their construction. In these structural characters there are therefore good and sufficient specific differences to discriminate the species. It is an interesting fact, but by no means a singular case, that we should have a fossil sponge from the hard chalk of Flamborough closely resembling the recent specimen, D. fistulosa, from Australia. This fossil sponge is described by M r . John Edward Lee in Charlesworth's 'Magazine of Natural History,' vol. iii. for 1839, p. 15, as Spongia spinosa, and is represented in page 16, figs. 11 and 12 ; and I have by m e a specimen of the same species of fossil from the locality mentioned by M r . Lee, which is m u c h more like the recent D. fistulosa than the specimens described and figured by that author. Not only does the fossil resemble the recent sponge in its external characters, but there is little doubt that in its living state its internal structure was also similar. Mr. Lee, in page 16 of his paper, writes, "Being anxious to see more of the internal structure, I had the specimen cut through just below the plates figured in the last diagram : an irregular fibrous structure then became visible, similar to that shown in fig. 12." The artist, in the figure of the specimen thus treated (13), has faithfully represented the fibrous structure alluded to, on each side of the lower part of the figure ; and the fibrous structure is so like that in the corresponding part of the recent D. fstulosa, that the drawing would equally well represent the arrangement of the fibrous skeleton-tissues of either species. |