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Show 1875.] SILICEO-FIBROUS SPONGES. 505 work of the dermis, we at once perceive that the spicula when thus united form a strong and elastic rectangulated network. This mode of combination of the simple sexradiate spicula is perfectly adapted to the power of dilatation and contraction that it appears should necessarily exist in all siliceous sponges, whatever their form may be, which have a rigid skeleton. W e find these powers existing in all the species of Dactylocalyx, as represented in plates v. & vi., P. Z. S. 1869. But in these cases the expansion of the dermis is effected by various forms of ternate spicula, connected by the apices of their terminal radii, while their shafts are directed towards the body of the sponge, so as to allow, not only of a great amount of lateral expansion and contraction of the dermal membrane, but also of the separation of the dermis from the body of the sponge beneath it to a very considerable extent. In all the species of Geodia and Pachymatisma we find the same principle existing under various modifications. The expansile powers of the dermal tissues are also provided for, in the reticulated structures of Isodictya and Halichondria, by the conjunction and elastic adhesion of the terminations of the spicula forming the dermal rete, whether that organ be raono-spiculous, as in many species of Isodictga, or multispiculous, as in numerous species of Halichondria and several other genera ; and where no such structures exist the dermal membrane alone is abundantly elastic, as exhibited in the protrusion of the large excurrent orifices in Spongilla, as figured in plate i. in the " Report on the Vital Powers of the Spongiadse," in the Reports of the British Association for 1857. All these beautiful appliances appear to be combined in the structure of the dermis of Alcyoncellum; and in addition we have the floricomo-sexradiate forms terminating the distal apices of the dermal expansible arrangement of spicula, as defences of the external surface of the dermal membrane against the minute enemies, while the chevaux-de-frise forms beneath are an ample and effective defence against the more powerful depredators. The spicula of the expansile dermal tissues vary in structure to a considerable extent in the different species of sponges in which they occur. In Geodia Dysoni they assume the form of simple patento-ternate spicula, their distal terminations being all in the same plane, their radii meeting and overlapping each other more or less, as represented in figs. 4 & 5, plate iii., P. Z. S. 1873 ; or they occur as bifurcated patento-ternate ones, as represented in figures 3 & 4, plate ii., P. Z. S. 1873, in the dermis of Geodia perarmatus. In the similar organs of Dactylocalyx Pratti their terminal radii are flattened and contorted to a considerable extent, as shown by figs. 9, 10, 11, plate v., P. Z. S. 1869 ; and in the same plate the radii of these spicula are expanded into beautiful foliations in the dermis of Dactylocalyx M'Andrewii, as represented in figures 2, 3, & 4. But however different their forms may be, their office in the expansile dermis of each sponge is precisely the same, and their long basal shafts are pendent, as represented in the section at right angles to the surface of Dactylocalyx Prattii in plate v. fig. 6a, P. Z. S. 1869. In all these cases the same design, with variations adapted |
