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Show 1 869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 69 Dr. Wyville Thomson's endeavour, by the institution of his proposed new order Vitrea, and his description of its characters, has, instead of elucidating the subject, still further complicated it. All the members of his new order should certainly agree in a series of definite structural characters; but this is not the case. Thus he gives, as examples of his order, Hyalonema and Dactylocalyx, the sponge of the first having a skeleton composed of spicula cemented together by keratode, as in the great mass of Halichondroid sponges, the mass of the skeleton being eminently elastic and entirely destitute of sihceo-fibrous structure, while in the latter genus the skeleton is perfectly rigid, being composed entirely of inelastic siliceo-fibrous tissue. But this is not the only error in the descriptions of the characters of his proposed new order Vitrea; thus he states (Ann. & Mag. Nat. Hist. p. 120) of the sarcode : - " It is small in quantity, very soft,' probably semifluid, extending in a thin layer over the fascicles of siliceous needles and over the siliceous framework." Dr. Thomson's description of the sarcode in this tribe of sponges is correctly applicable as regards quantity, if, as appears to have been the case, he has derived his conclusions from an examination of the prepared skeletons of the sponges in the Museum of the Jardin des Plantes and the British Museum ; but it is a mistake to imagine that the sarcode is deficient in quantity when in their natural condition. In Dactylocalyx heteroformis, D. McAndrewia, D. Prattii, and D. Masoni which are in the same condition as when taken alive from the sea, there is quite as much of that vital substance in their interstitial cavities as we find in the greater portion of siliceo-reticular sponges, and frequently more in proportion than we find in many of them. It is also an error to conclude that there are no membranous tissues within them. The usual delicate interstitial membranes exist in their cavities to about the same extent as in other sponges. The author also says, in his character of Vitrea, " never containing formed horny matter, either fibrous, membranous, or granular." Now Hyalonema contains an abundance of horny matter, cementing the spicula together in the basal mass of the sponge, aud also in the coriaceous envelope of the so-called " glass rope " of the sponge, and Dactylocalyx has plenty of membranes in the interstices of the skeleton, and an extensive and elastic dermal membrane enveloping the whole of the sponge. Prof. Thomson's description of the characters of his proposed new order embraces very many more genera than he could possibly have contemplated when he wrote it. He says, " The skeleton consists entirely of siliceous spicules, either separate (in fascicles or scattered) or anastomosing and combined into a siliceous network." Let us now see what the effect of this very sweeping character will be :- 1st, under the head of spicules " separate," it will include the genera Hymeniacidon and Hymeraphia; 2nd, " spicula in fascicles," it will embrace ten other genera, viz. Geodia, Pachymatisma, Ecionemia, Dictyocylindrus, Polymastia, Ciocalypta, Tethea, Phakellia, Microciona, and Hymedesmia ; 3rd, " combined into a siliceous net* |
