OCR Text |
Show 68 DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. [Jan. 28, effort to test its accuracy, as it appears to me to be impossible the fibres of Dactylocalyx pumiceus should be seen beneath a microscopical power of about one or two hundred linear without the conviction being immediately arrived at that the tissue was purely fibrous; and sections at right angles to their axes at once exhibit their concentric structure, and prove that they are not compound structures formed of "separate siliceous spicules of various forms, interwoven in fascicles." This description, quoted from Prof. W. Thomson's paper (Ann. & Mag. Nat. Hist., Feb. 1868, p. 120), will apply correctly enough to Hyalonema, but certainly not to " Euplectella, Owen, Aphrocallistes, Dactylocalyx, and Farrea," the latter four genera having purely siliceo-fibrous skeletons, while Hyalonema is as purely a spiculo-reticulate structure. Prof. Wyville Thomson, in his paper in the ** Ann. & Mag. Nat. Hist.' for Feb. 1868, has proposed a new name for the siliceo-fibrous sponges ; but a new name, unless it be more significant than the old one, is a detriment rather than an advantage to science. He designates them as vitreous sponges; this is an erroneous idea, inasmuch as the fibres are not inorganic and amorphous in their structure like fibres of glass, but, on the contrary, they are highly organized, consisting of concentric layers of silex and keratode combined, and thus are totally different in their origin and structure from an artificial amorphous structure like glass. The term vitreous naturally supposes an origin and a transparency through the agency of fire ; but if we submit the fibres of Dactylocalyx pumiceus to the action of that element by making a small portion of the rigid skeleton red-hot two or three times in the flame of a spirit-lamp, it comes forth from the trial as black as charcoal, and perfectly opaque. If the term vitreous is meant to represent the general character and appearance of these sponges in their natural condition, it is then still more inappropriate, as in the living state their external appearance is that of an ordinary sponge entirely enveloped in a more or less fleshy dermal envelope; if at all applicable, it can only be so when the animal is in a deteriorated and partially decomposed condition ; while siliceo-fibrous is correctly expressive of the nature of their structure, and contrasts well with the terms kerato-fibrous and spiculo-fibrous. For these reasons, therefore, I feel under the necessity of rejecting the new designation proposed by the learned Professor. Prof. Wyville Thomson, in his proposed arrangement of the Spongiadce " Order I. (P. silicea) Vitrea," gives the following as the characters of his proposed new order:-"Sarcode in small quantity, very soft; never containing formed horny matter, either fibrous, membranous, or granular. The skeleton consists entirely of siliceous spicules, either separate (in fascicles or scattered) or anastomosing, and combined into a siliceous network. The sarcode contains small spicula of a different character from the general spicules of the skeletons, and of complicated forms. The spicules, whether of the skeleton, or of the sarcode, may all be referred to the hex-radiate stellate type. Ex. Hyalonema, Dactylocalyx." |