OCR Text |
Show 1881.] THE SURVEY OF H.M.S. 'ALERT.' 113 an external framework, which is the main agent in giving the Sponge its firmness. Arising from this subdermal network, and generally closely enveloping it, is a much finer ("dermal") network or veil (corresponding in relations to the veil on which Schmidt based his genus Ditela, afterwards reunited to Spongia) ; its fibres are derived from the upper surface of the subdermal fibres by smaller branches, which branching out horizontally become much finer ; the finest form the finer part of the network, of which the coarser form the supporting ribs; the diameter varies from "02534 millim. for the coarsest to "00475 millim. for the finest fibres. The "veil," however, at the free end and at the lower edge of the Sponge projects beyond it as a loose envelope. The fibres are very transparent, of a pale amber-colour of various shades, and are delicately laminated. The vertical and smaller subdermal fibres are cored by a uni-serial row of fine acerate spicules, placed end to end. In the freely projecting parts of the veil the stouter dermal fibres may be cored by spicules quinqueserially arranged ; the finer dermal fibres are cored by uniserial acerates, although these are often wanting for considerable tracts, or only present at intervals. Parenchyma transparent. The spicules apparently sometimes occur singly or in groups in the stout subdermal fibres; possibly others have been present and been absorbed. SJceleton-spicules smooth, fine, acerate, tapering somewhat gradually to sharp points, nearly straight; size "0/0 by "001267 millim.; many of them have undergone more or less absorption. Flesh-spicules same as of skeleton. Examined. In the dried state and by mounting in balsam. Hab. Hotspur Bank, off east coast of Brazil (lat. 17° 32' S., long. 35° 46' W . ) , 35 fathoms, on piece of calcareous rock. Obs. As Schmidt's account is very short, and as the specimen is well preserved, the characters of the Sponge are given fully. The chief differences between this specimen and Schmidt's appear to be :-(1) the superficial (not axial) position in the stout subdermal fibre of the spicules in the former ; (2) the inferior diameter of that fibre as compared with the present specimen (being as 3 to 5) ; (3) the branching of Schmidt's specimen, and (4) its bearing (as appears from his referring to Duch. de Fontbressin and Michelotti's species) small spinous processes on its surface; and (5), lastly, the superior proportions of the spicules of Schmidt's specimen, which measure "0887 by 00211 millim. These differences justify the separation of this form at least as a well-marked variety, although our acquaintance with the Chalinidse appears to be too limited and their characters too few to admit of distinguishing it as a species at present. Attention is particularly called to the beauty and complexity of the arrangement of the skeleton. The above differences may be thus tabulated:- PROC. ZOOL. Soc-1881, No. VIII. 8 |