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Show 88 DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jail. 28, whole of its surface; they rarely exceed a line in diameter; the smaller ones are frequently simple orifices, the larger ones are slightly elevated and marginated. The pores are not visible without the aid of considerable microscopical power; with about 100 linear their structure exhibits an exceedingly beautiful appearance. They each occupy an area formed by the intermingling of the elegant foliations of the ternate connecting spicula ; and each little porous area is furnished with a regular fringe composed of a single series of the small dermal tension-spicula, which, projected from its margin inwards, meet at about the centre of the space, forming a complete defence against the incursions of any minute enemy ; in the dermal membrane around, the minute tension-spicula are closely and irregularly matted together (Plate IV. fig. 5). When we view a section of the sponge made at right angles to its surface, the structural peculiarities of the expansile dermal system of this tribe of sponges are very beautifully displayed. The outer surface is densely covered with the terminations of the ternate spicula of that organ, and again with the dermal membrane and its closely matted tension-spicula. Immediately beneath we see the pendent shafts of the ternate spicula, more or less clothed with minute acerate spicula, and with the proximal terminations of the shafts cemented by keratode to projecting portions of the fibre of the rigid skeleton, the surface of which is covered by a stratum of membranous structure, abundantly furnished with minute acerate spicula; the space between this surface-membrane of the rigid skeleton and the under surface of the expansile dermal system forms a large cavernous or crypt-like cavity supported by innumerable pillars at about equal distances from each other. The arrangement of the fibres of the rigid skeleton have all the complete irregularity of a Dactylocalyx, and there is not the slightest approach in any part to the confluent radial structure of an Iphiteon. There are a few comparatively large acerate spicula dispersed amid the reticulations of the rigid skeleton ; they are about four or five times the length of the dermal ones, and they are not frequently to be seen in situ. The connecting spicula are exceedingly beautiful objects. They are very variable in size and structure; and no two of them are alike in the mode of the foliations of their ternate radii, which are evidently modified to meet the necessities of the intermingling of their terminations, so as to secure a strong and elastic covering to the interstitial cavity beneath, and at the same time to produce abundant spaces for the porous areas of the dermis of the inhalant system. The structural aspect beneath the exhalant surface is very different from that of the inhalant one : here we find, as might be expected, large cavernous spaces for the reception of the effete streams from the rigid skeleton beneath, and, instead of the regular crypt-like form with its numerous minute pillars, we have elongated extensive spaces, the sides of which are, to a great extent, composed of irregularly disposed large acerate spicula imbedded in membranous structure; the shafts of the connecting spicula above are some of them connected with the parietes of the cavernous spaces, |