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Show 71 DR. J.S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jail. 28, to the membranes they are destined to support, and which, in well-preserved specimens, may be seen stretching from point to point of the radii. Fig. 2, Plate III., represents a spiculum of this form X 1 08 linear. In some species of siliceo-fibrous sponges there is a paucity or a total absence of the rectangulated auxiliary fibres and of the simple rectangulated hexradiate spicula; in such cases we frequently find their places supplied by numerous long acerate interstitial spicula dispersed in the interstitial spaces of the rigid skeleton, their office appearing to be to increase the surfaces of the nutrimental membranes. In specimens in which the animal matter is well preserved, the membranes are seen stretching from point to point of each spiculum, and from the points of one of these spicula to those of other similar ones in its neighbourhood ; and as these spicula occur grouped together frequently in considerable numbers, it may be readily conceived that they perform an important office in thus increasing the amount of the nutrimental surfaces within the animal. The incipient spination of the radii, so prevalent in this form of spiculum, admirably fits them to maintain their hold of the delicate interstitial membranes which are attached to them. The spinulo-trifurcated hexradiate stellate (Plate III. fig. 4) and other forms of those spicula appear to be peculiar to the siliceo-fibrous sponges. In the well-washed specimens they do not seem to be very numerous ; but in cases where the interstitial membranes are in a good state of preservation, they are occasionally found to be so abundant and so closely packed together as to completely cover and obscure the membrane beneath them. Occasionally the hexradiate stellate forms occur with the radii attenuated and acutely terminated (Plate III. fig. 5). This form of spiculum is abundant in the type specimen of Dactylocalyx pumiceus, and is probably either an abortive or an immature development of the spinulate form of spiculum. In m y examination of the corresponding forms of spicula in Iphiteon Ingalli I found two of this attenuating form of spiculum which, under a power of 108 linear, appeared to have their radii acutely terminated ; but on the application of a power of 666 linear I found that their apices exhibited incipient spinulation ; and these spicula were the only two that I could find, although I searched for other specimens in a similar condition amidst a crowd of spinulate ones which completely covered a fragment of membrane that I obtained from the sponge while in the possession of m y late friend Mr. Ingall. The situation and peculiarities of the oscula and pores afford important characters in the determination of the species in all sponges. In the cup-shaped siliceo-fibrous sponges the oscula are situated on the surface of the inside of the cup, and the pores on the outer surface. The same law obtains in the cup-shaped kerato-fibrous sponges of commerce and in numerous cup-shaped Halichondraceous species. In coating or amorphously massive sponges the oscula and pores occupy the same surface, and the incurrent and excurrent systems of canals are intermingled. The circulation of the nutrient |
