OCR Text |
Show 1873.] DR. J. S. BOWERBANK ON THE SPONGIADcE. 11 exceed an inch in diameter. Originally it had been nearly spherical in form; it was in the dried state when I examined it. W h e n alive it would probably have been about two inches in diameter ; and, judging its surface characters by those of the well-known British species T. lyncurium, which in many points it closely resembles in its anatomical characters, it would in the living state have presented a smooth surface, and in the dried state, as shown by the section figured and the microscopical sections in Canada balsam in m y possession, it would have been abundantly furnished with large depressed tubercles. The oscula and pores are not distinguishable ; nor could I detect the slightest indication of a dermal membrane. The dermal rind is more than usually thick in this species of the genus. The closely packed mass of the thick stratum of large sphero-stellate spicula, and their enveloping membranes, of which it is composed, would naturally prove a serious impediment to the inhalant operations of the sponge. To remedy this obstruction, the inhalation is effected through numerous very deep cylindrical depressions, which are found between the corymbose distal terminations of the skeleton-fasciculi ; these depressions form cylindrical sacs the depth of which is three or four times their own diameter, and extending nearly or quite to the inner surface of the dermal rind. Similar but more highly developed organs exist in several species of Geodia, as in G. Barretti and M'Andrewi, through which aerating and nutrient streams have access to the interior of the sponge. The skeleton-fasciculi radiate from a central mass composed of numerous loosely compacted bundles of spicula of the same form as those of the skeleton, but very much smaller; these fasciculi do not appear to have any definite arrangement, but cross each other in every direction. The skeleton-fasciculi proceeding from the central mass are large and closely compacted, and in this state they enter the inner surface of the dermal rind, and commence separating and radiating to form the corymbose terminations at the distal surface of the dermal rind of the sponge ; and they expand to such a degree that the large dermal tubercles thus formed are in very close conjunction on the surface. The stellate spicula of this species are very remarkable. The extreme diameter of a fully developed one of the largest description is •J-^-J inch, and the radii are, comparatively, exceedingly large and stout. The number of these organs is very great; and in the dermal rind they are so closely packed that the rays of each pass between those of the adjoining ones, and the whole become, as it were, cemented into a solid mass. The two smaller ones are also exceedingly abundant on the interstitial membranes. They occur in about equal quantities, thickly dispersed over the membranes. The largest of the two, the cylindro-stellate ones,have an average extreme diameter of ^y^y inch; while the smaller cylindro-stellate ones do not exceed -^TS *ncn m diameter, being less than one-third of the greatest diameter of a skeleton-spicu-lum, which measured TOV"? inch. |