OCR Text |
Show 1869.] DR. J. S. BOWERBANK ON SILICEO-FIBROUS SPONGES. 327 pound structure were incipiently spinous, but the internal ones were smooth. The portion of the compound fibre examined measured Jy inch in diameter, and is represented by fig. 2, Plate XXII., x 108 linear. The structure of the skeleton of Aphrocallistes beatrix, Gray, is precisely in accordance with that of Iphiteon panicea ; and if agreement in organic structure be an evidence of close alliance, the two must belong to the same genus, however different their external forms may be. The same description of symmetrical confluent areas of siliceo-fibrous structure forms the skeleton, the only difference being that the areas are rather less in their average diameter than those of I. panicea. In the latter species they average -^ inch, while in the former they are -fa inch; but in their general structural aspect they so closely resemble each other that, if it were not for the spinous umbonate centres of the areas in I". beatrix, they could not be distinguished when examined beneath the microscope. Fig. 2, Plate X X I . represents a section at right angles to the surface of the sponge. The view of the surface of the sponge does not exhibit distinctly the peculiar rotulate structure of the areas ; and it is only when we obtain a section at right angles to the surface that this strikingly characteristic structure is to be seen in all its symmetry and beauty. But the surface view exhibits many of the specific characters in an extremely striking and beautiful manner. Here we observe large inhalant spaces, abounding in rectangulated hexradiate spicula, for the support and multiplication of the nutrient membranes of the sponge ; and that the delicate tissues m a y be preserved from the ravages of minute annelids and other insidious enemies, the mouths of the apertures are abundantly defended by the projection into them of large elongate cones of fibre, profusely furnished with minute spines ; and deeply imbedded amidst the skeleton- fibre we find an abundant supply of acerate tension-spicula, and of the short, acerate, verticillately spined retentive ones, and occasionally groups of two or three of the porrecto-spinulo-quaternate spicula with attenuating shafts (fig. 3, Plate X X L , X 108 linear). The surface of the skeleton is furnished with a profusion of attenuated acerate external defensive spicula, the distal portions of which are abundantly spinous, the spines appearing as if notched upward out of the shaft of the spiculum, their acute points being all directed downward. The greater portion of these defensive organs are deeply immersed in the skeleton-mass beneath, their distal ends projecting not more than about one-fifth or one-sixth of their length beyond the general surface of the sponge. A section at right angles to the mass of the skeleton is necessary to exhibit distinctly their structure and position in the sponge. The auxiliary rectangulated hexradiate fibres of the skeleton are produced very sparingly in this species; they do not attain the full development of the shaft and lateral radiations as in Dactylocalyx, the lower half of the shaft only being produced; and this portion of it is abundantly spinous, and terminates hemispherically. In one portion of the skeleton, mounted in Canada balsam, their purpose iu the economy of the animal is dis- |