| OCR Text |
Show 90 DR J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jail. 28, rent to the outer portion of the rigid skeleton. When the back of this specimen was presented to the eye, this membrane was seen to be abundantly supplied with large, long, flat fasciculi of slender acerate tension-spicula. The minute short fusiformi-cylindrical spicula were as profusely scattered over the surface of this membrane as on the external dermal one. The porous system, especially when we view its inner surface, is a most beautiful object for the microscope. The interlacing radii of the large patento-ternate connecting spicula form a beautiful series of round or oval areas, each containing from one to four or five large pores, the greater portion of which were open ; and the dermal membrane ou which they exist is beautifully freckled with innumerable minute, entirely spined fusiformi-cylindrical spicula, so closely packed together as to completely obscure the surface of the membrane, while the acutely conical shafts of the connecting spicula are seen at regular intervals projected towards the eye. A portion of this beautiful membrane is represented by fig. 8, Plate V. The expansile dermal system is admirably displayed in this sponge by a section at right angles to the surface from almost any part of it. In some portions of such a section the dermal surface is closely pressed on to the surface of the rigid skeleton, while in others it is seen more or less separated from it, forming a cavity above it, into which the shafts of the connecting spicula are projected towards the surface beneath, as represented by fig. 6, Plate V. The irregularly furcated patento-ternate connecting spicula are singular in their structure, and very characteristic of the species. No two of them are precisely alike, either in size or form ; the eccentricity with which the radii are projected from the head of the shaft and the exceedingly variable mode of their ramifications are not a matter of chance, but they are evidently influenced by the necessities of their combinations with each other in forming the dermal network and porous areas ; for if we view them in situ, we observe no points straying from the lines of combination, but the whole of their radii are locked together so as to form a compact but expansile network for the support of the dermal membrane and the formation of the porous areas. The interstitial membranes filling the areas of the network of the skeleton are very translucent, and would scarcely be visible when immersed in Canada balsam, if it were not for the minute, short fusiformi- cylindrical spicula which are dispersed over their surfaces. These spicula, though exceedingly minute, afford very decisive specific characters. They are dispersed, more or less, over every part of the membranous structures, but more especially on the dermal membrane and the enveloping membrane of the rigid skeleton, which tissues they completely cover. They require a power of from 700 to 1000 linear to define their structural characters in a satisfactory manner. They vary considerably in size ; one of the largest that I measured was ^aVs m c ^ m length, and yy^-ge inch in diameter; one of the smallest measured jjfeg inch in length, and g-ijuTnT m c n in diameter. |
