OCR Text |
Show 30 DR. J. S. BOWERBANK ON HYALONEMA MIRABILE. [Jail. 10, two together ; but occasionally they occur in groups of ten or twelve, without any approach to a definite arrangement (Pl. V. fig. 1). These spicula are comparatively large and stout. They have eight rays at each end of the shaft, the two groups of radii curving towards each other to such an extent that each forms the half of a regular oval figure, the opposite apices being separated to the extent of about the length of one of the radii. Each ray is in form like a double-edged obtusely pointed knife bent near the handle in the direction of a line at right angles to the inner surface of one of its flat sides; and each ray is strengthened and connected with the shaft of the spiculum by a stout curved web of silex, which extends from a little below the inner surface of the ray to a point on the shaft about opposite to its middle. The edges of each ray are also slightly curved inward (Pl. V. fig. 2). The smaller or secondary system of birotulate spicula differ somewhat from the larger ones in structure. They are not fimbriated at the base, as those of the larger ones are, nor have the radii the same distinct cultelliform figure (Pl. V. fig. 3). Their position in the sponge is also different. The larger ones are always irregularly dispersed ; while those of the smaller system are usually congregated in considerable numbers around the large skeleton- fasciculi, their direction being coincident with the axial line of the fasciculus (Pl. V. fig. 4) ; a few, comparatively, are dispersed, but this mode of position appears to be rather the exception than the rule. The shaft is cylindrical, and has short stout tubercles dispersed over all its parts, and the radii are so long in their proportions that the opposing apices very nearly touch each other. The third form of interstitial spiculum, the cylindro-cruciform one (Pl. V, figs. 10, 11, 12, 13, 14), appears to appertain more especially to the cloacal system ; they are found abundantly dispersed near the inner surface of the coriaceous dermis of the spiral column of the sponge ; but they occur in by far the greatest number between the basal portion of the spicula of the spiral column, and in their immediate neighbourhood, intermixed with the large hexradiate spicula of the interstitial tissue of the great basal mass of the sponge; and at the top of this mass tbe spiral column is surrounded by a profusion of them. The radii are short and very stout in their proportions, their length varying from twice to five or six times their own diameter; and the four rays are frequently of different lengths. They are profusely covered with large, stout, more or less conical spines, and especially so at their distal extremities. In all these characters they vary to a considerable extent even in the same group. They appear to be more matured in the basal portions of the sponge than in the coriaceous dermis of the spiral column; in the latter position they are frequently represented by short, stout, entirely spined cvlindrical spicula (PL V. fig. 10) ; but between this rudimentary state and the completely cruciform spiculum specimens may be found in every intermediate stage of development. Occasionally a spiculum may be found with a fifth ray, indicating that the cruciform spiculum is in truth only a modification of the regular hexradiate type of the inter- |