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Show 1869.] DR. J. S. BOWERBANK ON SILICEO-FIBHOUS Sl'ONGES. 341 late spicula, with which it is probable that the membrane was amplv furnished as secondary defences against minute enemies. This singular tissue is figured in the ' Philosophical Transactions of the Royal Society' for 18C2, plate 32. fig. 7, and also in my 'Monograph of the British Spongiadse,' vol. i. plate 21. fig. 311. I believe the portions presented to the eye in the pieces figured to be the external surface, as the fragments of the dermal membrane which remained all seemed to cover that side of the fibres of the network, and the presence of the external series of the spicular organs is strongly indicative of the minute hispidation of the surface of the sponge in its natural condition. In the present condition of the sponge it is impossible to determine whether this singular harrow-like dermal structure was continuous over the whole of its surface when in the living condition ; but the probability is, judging from the general structure of the expansile dermal system of every other known species of siliceo-fibrous sponge, that it was composed of detached sections, so as to allow of the usual amount of expansion and contraction that we observe to exist in every other such sponge. The reticulation of the skeleton is always angular, but the areas vary from square into all imaginable varieties of the oblong figure. The fibre is stout and strong, with a well-defined central canal in its fully developed condition; a portion of it is represented in Plate XXIV. fig. 1, with numerous attenuato-stellate retentive spicula adhering to the fibres. Occasionally in some portions of the skeleton-fibre we find two canals, neither of which are central: this abnormal form probably arises from two immature fibres, closely approximated in an early stage of their development, uniting longitudinally; and in one case I observed as many as three irregular portions of canals in one fragment of the fibre ; but this irregularity of structure is the exception and not the rule. The spination of the skeleton-fibres is very slightly produced in the form of acute cones, and in some of the larger fibres it may be almost designated as incipient, while occasionally in some of the immature ones the spinules assume the forms of tubercles, which are sometimes more or less bifurcated. The interstitial tension-spicula of this sponge are very remarkable organs. They are simple biternate, spiculated biternate, and furcated spiculated biternate. Sometimes one termination only is spiculated, sometimes both are thus furnished. One or two of the terminal radii are frequently furcated; but it is of rare occurrence that the whole of them are produced to that extent. They occur in groups entangled together; in several of these groups they were numerous and closely packed, much in the same manner in which we find the spinulo-trifurcated hexradiate spicula of the interstitial membranes of Dactylocalyx pumicea when seen in situ. They are stout and comparatively of large size (Pl. XXIV. figs. 5 & 6). The attenuato-stellate retentive spicula are minute and very irregular in their structure and in the number of their radii. They have evidently been very numerous, as they are frequently found adhering PROC. ZOOL. Soc-1869, No. XXIII. |
