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Show 1869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 349 liarities of the animal. Thus the true natural positions and mode of arrangement of the stout attenuated rectangulated hexradiate spicula, the full series of the varieties of which are figured in the ' Philosophical Transactions,' 1858, plate 25. figs. 24-33, and in Mon. Brit. Sponges, vol. i. plate 7. figs. 174-183, are well exhibited in situ, which I have never yet seen in any of the well-washed specimens with which we are now so familiar. In the large lateral orifices of such specimens they are sometimes entirely wanting, or a few only of them are found in the neighbourhood of the large circular area. In the specimens in which they are held in their natural positions by the sarcodous and membranous tissues, they are regularly disposed around the circular area, forming a compact marginal ring, their stout radii projecting in every direction among the surrounding portions of the skeleton, but not within the circular area; so that where one of the radii would, by the natural laws of development, have been found, its production is arrested, and it is represented by only a slight tumefaction on the axis of the spiculum ; hence it is that we find such numerous varieties of form among these remarkable spicula. All the other radii immersed in the surrounding structures are completely developed, crossing each other in every direction; so that although unconnected by siliceous cementation with the fibrous skeleton, they form a strong but somewhat expansile marginal band to the circular area. W e are thus enabled to perceive the reason of the numerous cases of the suppression of frequently several of the radii of these marginal spicula, and to read the important fact from their positions and modifications that their production is as much regulated and modified by the structural necessities of the organs in which they form important parts, as are the bones and other organic structures of the most highly constituted animals. The true positions of the slender rectangulated hexradiate spicula with elongated basal axial rays are also well determined in these specimens ; they are seen in considerable numbers in the interstitial cavities of the sponge, supporting the interstitial membranes, and vastly increasing the amount of surface in those vital organs. The trifurcated attenuato-hexradiate and floricomo-hexradiate spicula are not very numerous; they are irregularly dispersed on the sarcodous membranes of the sponge, and are completely immersed in the sarcode, and without the aid of Canada balsam are usually invisible. DESCRIPTION OF THE PLATES. PLATE XXI. Fig. 1. A portion of the rigid skeleton of Iphiteon panicea from the specimen from Porto-Eico, in the Museum of the Jardin des Plantes, Paris, exhibiting the confluent structure of the rotulate areas of the skeleton, rectangulate hexradiate spicula, and a few gemmules in situ, x 108 linear. Fig. 2. A section at right angles to the surface of Iphiteon beatrix, exhibiting the confluent rotulate structure of the rigid skeleton, the fasciculi of acerate spicula, and the verticillately spinous retentive spicula in situ, X 108 linear. |