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Show 1868.] DR. J. S. BOWERBANK ON SPONGES. 123 the way of the student, who would naturally conclude they were organs always present in the sponge, while, in fact, he may search for them in many dozen specimens of Halichondria panicea and other allied species without once seeing a gemmule in situ. If we may judge of the extent of their distribution by their ascertained presence in various genera, and by the negative evidence afforded by our knowledge of other descriptions of ovaria, it will not be an overestimate if we believe that they will be found in at least 95 or 96 per cent, of the known siliceous and keratose sponges. Page 502, subsection 1. " Netted sponges (Dictyospongiee). Skeleton formed of a continuous siliceous or horny network." This definition embraces so wide a range and such varied modes of structure that it is calculated rather to bewilder the student than to facilitate his researches. It would include rather more than 75 per cent, of the whole number of the British sponges, and also the whole of the author's second, third, and fourth orders, excepting his Tethyadee. Subsection 2, p. 503. The families included in this section, if distributed in accordance with its wording, might just as correctly be referred to subsection 1, page 502, as to order 4, p. 504, section 2. Chlamydosporee is founded on a misapprehension of the structure of the spicula of the ovaria, which have no defensive characters, but are simply portions of the wall of the ovarium, which is so constructed that the spicula may allow of a slight degree of lateral expansion of that body ; their external surfaces in the adult state are flat coincident planes. The sponges having truly armed ovaria or gemmules are entirely excluded by the author from this section, the family Tethyadee being disposed of in his order 4, Acantho-spongia, p. 504. In truth, the confusion of ideas existing in the descriptions of these orders is such as to render it perfectly impossible to comprehend the characters by which species are to be referred to any one or the other of them. This course of proceeding is not divisional arrangement; it is no separation of the multitudes with the members of which we want to cultivate an individual acquaintance. These vague expansive ideas are not definitive or distinctive. What we want in such descriptions is a certain group of characters within a definite circle to be readily separated from other well-defined groups or circles by which they are surrounded, such as we see amidst the natural orders of plants, and not a flow of indefinite verbosity, that leaves us in a labyrinth of words, amidst an impervious cloud of ideas. The Orders. The author's orders are seven in number. They are formed principally by an elevation of his own and other writers' genera to that dignified position in science, their characters being constructed by a free version of their previous generic ones ; but in many instances these alterations or additions have a very unfortunate result, as they have led to serious errors of description. |
