“19. Aplysina membranosa.
Spongia membranosa, Pallas, Elench. Zooph. p. 398.
Spongia membranacea, Esper, Pflanzenth. ii. p. 256, pl. xxxiv.
? Ianthella concentrica, Hyatt, Mem. Bost. Soc. ii. p. 407.
? Aplysina purpurea, Carter, Ann. & Mag. N. H. (5) vi. p. 36.
A very fine species, fortunately preserved in spirit as well as in the dry state. It forms a cylindrical mass, which has the same general form as that given in Esper's figure (1. c.); the base of the wet specimen is about 40 millim. (1 3/5 inch) in diameter. The dry specimen, which is much the largest of the two, is 580 millim. (22 ½ inches) high, and the maximum diameter, which is at about 5 inches above the base, is about 40 millim. (1 3/5 inch). At 77 millim. (3 inches) above the base a branch is given off, 22 millim. (7/8 inch) in maximum diameter and 145 millim. (9 ½ inches) in length, and on the same side, about 30 millim. higher up, a smaller branch, 12 millim. (½ inch) in maximum diameter and 75 millim. (3 inches) in height. The skeleton consists of a very open and irregular network of fibres, 1.5 to 2 millim. in thickness, which arise at the base of the sponge, and take a longitudinal but somewhat sinuous course along the interior of the cylindrical column of which the sponge consists. They throw out branches somewhat freely from their sides, and subdivide terminally into ramifying branches; the resulting twigs anastomose freely, the superficial ones end in outwardly and upwardly directed points, usually bi- or tri-furcate, which are just covered by the tough dermis; the apices are .18 to .28 millim. in diameter. The dermis and the internal membranes consist of a tough membrane of a puce or dull purple colour in spirit, almost black in the dry state. The membrane is seen with the naked eye to be marked with numerous raised thickened lines, which radiate from the projecting apices of the dermal conuli (formed by the tension of the dermis over the points of the skeleton, as mentioned above), and branch and anastomose on the membranes. Under the microscope they are seen not to be special fibre-structures, but to consist simply, of thickened membrane. The membrane is coloured by purplish cells, which are about .03 millim. in diameter, and are crowded with semiopaque granules, to which they owe their colour. The fibres of the skeleton have a wide central cavity, occupying about half their diameter, and filled, or almost so, with a transparent substance coloured diffusely of a purplish-red colour. The walls of the fibre are composed of laminæ which separate readily, and may then be seen to consist of a dark substance, rather readily torn, thickly set with fine dark purple-red granules, lying in a diffusely stained subtransparent matrix of the same colour, but paler. A transparent membrane, consisting of an almost colourless matrix, containing few purple granules, appears to invest the fibre.
Hab. Thursday Island, Torres Straits, 4-5 fms.; bottom sand, or sand and rock.
Distribution. “Indian Ocean” (Pallas).
Obs. In many particulars this species recalls Aplysina purpurea of Carter, but appears to differ fundamentally in the distinctness, large size, and non-multiplicity of the fibres ; whereas in that species the axes of the conuli and the skeleton generally consist of aggregated masses of fine fibrils. If Hyatt's species is really like Ianthella homei, with which he compares it, it cannot be this sponge, as it would be of flattened growth ; but he appears to be uncertain on the point.
If one of the dermal cones, with the surrounding membranes, is treated with a strong solution of caustic potash, a dark brownish-yellow colouring-matter is dissolved out, thereby differing from that of Ianthella, which is said to be violet under similar circumstances (Gray, Proc. Zool. Soc. 1869, p. 50); nothing of the tissues is left but a branched fibre or two and some flocculent matter ; therefore the only truly fibrous structures here are the terminal twigs of the skeleton.
The wall of the main skeleton-fibre of this species is much thicker than in most Aplysina, and its axial substance is not granular.”
(Ridley, 1884)
