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Show 1869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 345 fig. 5. These strongly marked points of resemblance in form and identity in relative situation and office between the auxiliary spicula, in addition to those of the skeleton, irresistibly lead us to the conclusion that these sponges, however different in their forms, are structurally members of the same family. Strongly marked differences in form are apt to lead our judgments astray when superficial observations only are made of the specimens before us; but when we see such extraordinary variations of form occurring in the same species under different circumstances and amounts of development as those we observe in sponges with the habits of which we are perfectly familiar, as, for instance, in our protean species Halichondria panicea, we should be prepared to admit, as in truth we ultimately must do, the same latitude of variation among the nearly allied species and individuals of the same species of the siliceo-fibrous sponges. In all the numerous specimens of Alcyoncellum with which I am acquainted, the skeleton is composed of rigid inosculating siliceous fibre, as I have stated in m y paper on Alcyoncellum speciosum, Proc. Zool. Soc. 1867, p. 351, in m y description of the generic character in p. 353, in the following terms: - "Skeleton siliceo-fibrous ; primary lines radiating from the base in parallel, straight, or slightly spiral lines ; secondary lines at right angles to the primary ones." I will not reiterate here the full details of the structure of these beautiful sponges that I have given in m y paper as quoted above ; and such a repetition is the more unnecessary as they have been imported so abundantly of late as to place specimens for microscopical examination within the reach of almost every one interested in the subject. The sponges have also been figured in Trans. Zool. Soc. vol. iii., and in Trans. Linn. Soc. London, xxii. pl. 21, and also in the Ann. & Mag. Nat. Hist, for Feb. 1868, pl. iv. ; but in none of these plates is there any delineation of the skeleton-structure with a high microscopical power, and it is this want that I purpose at the present time to supply, that we may be enabled to arrive at a sound conclusion as regards its true skeleton-structure, and also as to such of its specific characters as have not hitherto been figured or described. Dr. Gray, in his " Notes on the Arrangement of Sponges," Proc. Zool. Soc. 1867, p. 492, has, at p. 504, described the Euplectelladee (Alcyoncellum, Quoy et Gaimard) as having a " skeleton composed of longitudinal, transverse, and oblique bundles of spicules intersecting each other and forming a network ;" and Prof. Wyville Thomson, in his paper on the "Vitreous" Sponges, Ann. & Mag. Nat. Hist, for Feb. 1868, p. 114, at p. 126, in his description of his proposed new genus " Habrodictyon," has adopted the error into which Dr. Gray has fallen by describing the skeleton as consisting " of a perfectly irregular network of siliceous needles loosely and irregularly arranged in sheaves crossing one another at low angles, and connected by a small quantity of soft mucilaginous sarcode." These descriptions of the skeleton are, in both cases, completely erroneous, as can be readily demonstrated by boiling portions of the |
