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Show 356 DR. BOWERBANK ON ALCYONCELLUM SPECIOSUM. [Mar. 28, ridge is coeval with a very early condition of the animal, and that the increment of the sponge has taken place in the space existing between its base and apex. In the type specimen formerly in the collection of the late Mr. Cuming, a few ridges such as occur on its outer surface were apparent on its inner one, a little below the oscular area ; but I have not detected them in the corresponding situation in the specimens in m y own possession. The oscular area within its beautiful circular frill or ridge is entirely filled with oscular orifices. The network of which it is formed is simple and irregular in its structure ; the rete is composed of numerous closely compacted fibres, so arranged as to afford the greatest amount of resistance in the least possible space. A transverse section of one of these fibres would be like that of a double convex lens. The excurrent orifice of this great terminal network is well indicated by the absence of interstitial spicula within its area, although on the inner surface of the oscular ridge bounding it they are in as great abundance as on other parts of the body of the sponge. In the living condition there is little doubt that the whole of the area would be furnished with a stout dermal membrane containing the true oscula of the sponge. In one of the areas of the oscular network of one of m y specimens, near its margin, I found a fragment of such a membrane, about an eighth of an inch in diameter, in a fine state of preservation. It was furnished with a reticulation formed of numerous long acerate spicula closely fasciculated together ; and in conjunction with this network there was a layer of sarcodous membrane, in which several of the well-known forms of the interstitial spicula of the sponge were imbedded, thus verifying the reticulated structure as a portion of the tissues of Alcyoncellum ; but the semidetached state of the fragment forbids our assigning it with certainty to the dermal tissues of the sponge. The sarcode is abundant on this fragment; and, as in other smaller fragments of that substance which I have found adhering to parts of the skeleton, the colour is that of a full amber-yellow. The series of equidistant circular apertures disposed in single lines between the primary lines of the skeleton are evidently the inhalant areas of the sponge ; and above each of these in the living condition there is most probably a congregation of pores, like those above the intramarginal cavities in Geodia and Pachymatisma. The margins of these apertures consist of a stout ring of siliceous fibres, very like those at the summits of the diagonal ridges on the body of the sponge. The interstitial spicula of this sponge afford a numerous and beautiful series of objects for the microscopist; and some of them appear to be peculiar to this species. In well-preserved specimens of the sponge that have not been washed and bleached to make them look pretty, they are so numerous and so closely packed together that in some parts they entirely obscure the view of the skeleton-lines beneath them ; and, if we may judge by analogy, their office is to afford points of adhesion to the interstitial membrane, and thus to vastly increase the amount of surface of the nutrient membranes and sar- |
