OCR Text |
Show 1881.] THE SURVEY OF H.M.S. 'ALERT.' 123 drical spicules, which, with loose ones of the same kind, form the dermal skeleton by the crossing of their ends. Main fibre, spicules united somewhat loosely. Parenchyma compact, almost perfectly colourless and transparent. Skeleton-spicules of three kinds, viz.:- (i) acuate, covered with sparse, short spines for about 4 diameters, from base, tapering to point gradually, size "1647 by "006334 millim., forming basal portion of vertical fibres ; (ii) acuate, smooth, head almost pointed, apex generally somewhat abruptly pointed, size '196 by 006334 millim., forming median portion of vertical fibre; (iii) cylindrical, smooth, of mainly uniform diameter throughout up to the heads, which are slightly swollen, and then end in more or less sharp hastate points, size "1774 to "18736 by "0038 millim., forming basal and dermal skeleton and summit of vertical fibres. Flesh-spicules, besides the last-named, fine acuates, very slightly blunted at base, tapering to very fine apex; size '152 by '0009 millim. ; scattered universally through sarcode. Examined. In spirit and by mounting in balsam. External Characters. The single specimen, which is extremely well preserved, coats the valves of a Pecten, which was alive when taken. It forms a thin film, varying in thickness from about "70 millim. to tissue-paper thickness. It fills up the depressions between the ribs, and thus presents a very smooth rounded contour ; but the lens shows that it is covered with minute points and shallow pits, the former probably representing the terminations of the primary skeleton-columns. Vents are possibly represented by two or three irregular depressions or openings, "25 to "5 millim. in diameter, near the thickest part of the sponge. Pores not found. The skeleton is simple in structure, and represents the type assigned to Hymedesmia by Dr. Bowerbank. The basal lamina is composed of loosely aggregated spicules, about 3 or 4 spicules thick. The composite structure of the vertical fibre is remarkable, and well adapted to secure, by the spination of the basal spicules, solidity of rooting, and, by its shading off into less stout spicules above, pliability. No special cementing sarcode is apparent. The structure of the dermal skeleton is essentially that of the Tedanice, though its connexion with the main skeleton is more marked than is usual in that genus. The parenchyma is slightly yellow, but in the almost entire absence of colour and of opacity resembles that of the Renieridse in general. The two larger skeleton-spicules (acuates) are probably varieties of one original type ; from its position, at the base of the columns with the roughened end downwards, the spined acuate is perhaps developed to suit the incrusting form of the Sponge; otherwise it differs from the smooth form mainly in being slightly shorter. The cylindrical are really sharply pointed ; but the penultimate swelling is generally discernible, and sometimes gives a fine spear-head outline to the head; they seem to be a further development of the typical cylindrical form in the same direction as that shown by Tedania tenuicapitata (sp. n.). Hab. Portland Bay, Chili (opposite the chief island of Madre-de- Dios Archipelago), 10 fathoms. O n both valves of small Pecten. |