| OCR Text |
Show 1869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 79 seum in about the same degree of preservation as the type one; but in consequence, probably, of not having been so much washed to make it look pretty, it abounds in the beautiful and characteristic spinulo-trifurcate hexradiate stellate retentive spicula. The fibre in the skeleton is abundantly but irregularly tuberculated, as represented in fig. 1, Plate III., from a section of the type specimen from Barbadoes in the British Museum, x 108 linear. The tuberculation of the fibre is remarkable and very characteristic ; when viewed with a power of about 700 linear, their apices are always more or less papillous ; in some the papillse are numerous and well produced, while in others they are in an incipient condition. Fig. 13, Plate III., represents two of the tubercles on the side of a portion of skeleton-fibre with their terminal papillae, X 666 linear. Beside the large primary fibres, there is a secondary series of skeleton-fibres, which are evidently auxiliary to the larger system. They occur especially in the large interstitial spaces of the sponge, their office being apparently that of filling up those vacant spaces when no longer necessary in the economy of the animal, and to sustain therein the multiplied folds of the interstitial membrane ; their office in this respect is the same as that of the large rectangulated hexradiate spicula (Pl. III. fig. 2) which occur so frequently in the interspaces of the skeletons of the siliceo-fibrous sponges, and their mode of development very closely resembles that of those spicula. In an early stage of their growth they very closely simulate the form of the spicula ; but instead of being freely developed amidst the membranous tissues, they are always based on the primary skeleton-fibres. A single small fibre pullulates from some part of one of the larger skeleton-ones, and is projected in a straight line into the vacant space: if it meets with none other in its progress, at some distance from its origin four lateral branches are thrown out at right angles to the axial fibre and to each other, and the axial fibre continues its progress in a straight line. If it meets no other fibre in its progress, the distal ends of the axial fibre and of the lateral ones become clavated, and all parts of the shaft and radii profusely spinous, and the whole constitutes a perfect simulation, in form, of a rectangulated hexradiate spiculum. But, on the contrary, should the axial or the radial branches meet with other such fibres, they immediately inosculate, and the previously straight radii are contorted in various directions to meet the necessities of the situation ; and, as is frequently the case, where many of these fibres are projected from different bases into the same space, they unite and form one mass of small contorted fibres, while there is good reason, from the gradual increase in size of the basal portions of some of them, to believe that they are ultimately developed into the size and form of the primary skeleton-ones. The primary skeleton-fibre averages -^ inch in diameter; the auxiliary fihres vary from 3 - ^ to 3 - ^ i n c h in diameter. How ever closely* they may simulate the form of true hexradiate spicula, they 'may always be distinguished from them by their attach- |
