OCR Text |
Show 504 DR. J. S. BOWERBANK O N [June 15, conformable, both as regards each other and the large primary fibres of the skeleton; and I could not detect a single floricomo-sexradiate one among them. The simple rectangulated sexradiate spicula, when immersed amid the skeleton-fibres, usually had the whole of the six radii developed; but those at the inner margin of the great incurrent areas were frequently deficient of the distal axial ray, so that the four lateral rays were presented in the same plane as that of the inner margin of the great incurrent areas. On examining the outer surface of the specimen we find a marvellously beautiful compound reticulated dermal skeleton elevated slightly above the general surface of the sponge-structures beneath. This exquisitely beautiful tissue is composed of a single layer of slender, simple, rectangulated sexradiate spicula, conformably arranged in the same plane, the long proximal ends of the central shafts being all pointed downwards towards the skeleton beneath, while the lateral radii of each spiculum glide over those of their next neighbours until the distal ends of their respective rays closely approach to the central shafts of each other, thus systematically interlocking with each other, and forming a beautiful quadrangular network in the dermal stratum. The distal portion of the central shaft of each simple rectangulated sexradiate spiculum has a single floricomo-sexradiate spiculum cemented to its apex, forming a uniform stratum at regular distances of these beautiful objects immediately beneath the outer surface of the dermal membrane. The interstices of the quadrangular network are filled by a thin translucent membrane on which there are occasionally found, closely adhering to the membrane, groups of five or six minute quadrifurcate sexradiate stellate spicula, very similar in general structure to those of Iphiteon Ingalli, figured in the Society's 'Proceedings' for 1869, plate xxiii. fig. 2, p. 331--but with this difference, that the radii of those oi I. Ingalli are spinulate, while those of A. speciosum are attenuated to exceedingly sharp distal terminations. These spicula are very slender and delicate in their proportions, and require a power of at least 400 linear to render them distinctly to the eye. A fully developed one measured as follows:-extreme diameter T^-g inch; diameter of the sexradiate basal portion Y5V0 m c n ; length °f the furcating radii Yifa-$ inch ; and the diameter of the thickest portion of furcating radii y^xf-fny inch. Thus these beautifully constructed and elaborately arranged organs form most effective defences against minute annelids or other insidious enemies who may attempt to prey upon the soft gelatinous tissues of the sponge. A single mouthful of the minute sharp-pointed spicula of which their beautiful floral terminations are constructed would effectually deter these predacious little enemies from any further attacks upon the soft tissues of the sponge. A more complete or more effective mode of disposition of these wonderful defensive organs cannot possibly be conceived. These structures are amazingly beautiful to our eyes ; but their admirable adaptation to their especial purposes infinitely surpasses their beauty in our estimation. To return to the peculiar mode of construction of the rectangulated net- |