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Show 136 DR. J. S. BOWERBANK ON SPONGES. [Feb. 13, The nature of these characters is not that of organisms indispensably necessary to the existence of a sponge, but rather of those of an auxiliary description, which one species may require while another may well dispense with them. As our knowledge of species extends, we quickly find that there are groups existing among them marked by peculiar modifications of the more essential organs of the sponge, such as those of the various modes of arrangement of the mechanical structure of the skeleton, while the material of which it is formed remains the same through a considerable number of such groups. Thus we arrive by a natural process at generic groups, which are on this constructive anatomical system quite as natural as species. Hitherto we have had but few well-defined genera of this description; and these few have been, as it were, thrust upon us by the irresistible evidence of Nature herself. Contrary to the received opinion of many able naturalists, I firmly believe that genera are quite as natural as species, and that it only requires perseverance on our own parts to recognize them, and to define their boundaries with accuracy and certainty. Of course we must expect cases occasionally to arise in which they approach each other by such insensible degrees as to render it extremely difficult to distinguish between the approximating points ; but this appears also to be the case in botany, and in other branches of natural history, and may be considered the exception and not the rule of nature. Having thus established a natural foundation to our system of genera, the next step is that of classification; and here the material of which the skeleton is formed becomes a most important point of consideration. W e find in the higher classes in zoology the animals of the highest degrees of organization secreting phosphate of lime as the basis of the skeleton ; and as we descend in the scale of creation, the phosphate is replaced by carbonate of lime, and the skeleton becomes external instead of internal; the next step downward, and we lose the earthy material, and the kera-tode skeleton supplies its place; membrane succeeds to keratode ; and finally all these substances are wanting, and sarcode becomes the entire animal. But amidst all these changes we find no class of animals secreting silex as the basis of its skeleton; this material is reserved as the great distinguishing character of the class Protozoa. Among the Spongiadse we observe a natural simulation, as it were, of this gradational retrogression of organization. None of them secrete phosphate of lime ; but the highest organized species secrete carbonate of lime in abundance. The next gradation is the secretion of silex in place of lime, the silex being the predominant material of the skeleton; and to this tribe belong by far the greater number of existing sponges. Then follows an intermediate stage, similar to that of the cartilaginous fishes, where we find the cartilage the predominant material, with included floating or dispersed portions of earthy basis ; so in the sponges, as in Chalina and other genera, we have keratode the essential portion of the skeleton, with siliceous spicula immersed in the fibres to give additional strength and substance to the Structures ; and, finally, the skeleton becomes pure keratode, without |