OCR Text |
Show 1869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 81 structures, I treated the remaining portion of the specimen by boiling it in nitric acid, and obtained not only numerous specimens of the spicula I have described above, but others of au exceedingly interesting description, which I shall now proceed to describe. The large verticillately spined spicula are very numerous, and exceedingly various in their proportions. They are usually more or less curved, and vary greatly in size and in the mode of their spination : some of the larger ones are acerate; that is, each end terminates in a well-produced point; others have at one end an irregular aggregation of stout spines, while the other is acutely terminated; and in some both ends are crowded with stout spines; and the general character of the shaft is that of a cylindrical spiculum. They occur in every imaginable stage of development, from extremely delicate diameters with the whorls of spines in quite an incipient condition (Plate III..fig. 6 a) up to the fully developed spiculum (fig. 6 b). The number of whorls of spines vary from 9 to 16; one with the latter number measured -gL- inch in length, and the diameter of the shaft was -^j inch. The spines are large, acutely conical, and there are seldom more than five or six in each whorl. These spicula must have been very numerous and closely disposed in the membrane. The two small pieces acted upon by the acid would not have exceeded the space of a quarter of a superficial square inch, while the results of their dissolution by the acid would cover more than a superficial square inch, and in a microscopic field of view -fe inch in diameter I counted as many as twenty-one of them. Under all these circumstances there can be no reasonable doubt of these spicula being those of the defensive system of the dermal membrane of the sponge ; and such spicula are usually found as abundant in the basal membrane as in other parts of the dermal system. I found also a considerable number of small equiangular or sub-inequiangular triradiate spicula with smooth attenuated radii, varying in size, from point to point of the rays, from ^fa to -^^ inch (Plate III. fig. 7). Such spicula are usually comparatively few in number, and are dispersed irregularly on the surfaces of the dermal or interstitial membranes of sponges. At the margin of a fragment of the sponge from very near the basal attachment, which was mounted in Canada balsam in its natural condition, I found the small equiangular spicula and little acerate ones (Plate III. fig. 8) imbedded in the membrane amidst minute attenuato-stellate ones. In this position they may therefore be regarded as tension-spicula of the dermal membrane. Amidst the other spicula resulting from the dissolution of the fragments from the base of the sponge by nitric acid there were several furcated attenuato-patento-ternate (Plate III. fig. 9) and dichotomo-patento-ternate (fig. 10) connecting spicula. One large one of the last-named form measured across its ternate termination Jg inch ; and all of them had large central canals in their radii. These spicula appear to vary considerably in size ; a smaller one measured - ^ inch in greatest expansion. There can be no doubt that they belonged to the expansile dermal system of the sponge ; and the small number of them found may be accounted for by their PROC. ZOOL. Soc-1869, No. VI. |