| OCR Text |
Show 8 PROF. E. A. MINCHIN ON THE [May 2, ray is usually 8 or 9 ju, but may reach 12 /u ; speaking geneia y, slender triradiate systems, with rays not exceeding M. 111 breadth, can be distinguished from thick ones with rays excee mg 10 n (text-fig. 3, 2 a -2/). In some specimens the triradiate systems are all, or nearly all, of the slender type ; in others, triradiate systems of the thick type are more abundant. Some of the triradiate systems develop gastral rays, becoming quadriradiates, and others do not. As a rule the quadriradiates are more abundant than the simple triradiates. In some specimens there is a tendency for the simple triradiates to be of rather stouter build than the quadriradiates, but in other specimens this cannot be noticed. The gastral rays of the quadriradiates are attached at the centres of the triradiate system, and are remarkable for their slenderness and usually also for their length (text-fig. 2, 1 g- 1 i). Arising from a slightly expanded base, the gastral ray sometimes tapers rapidly to a point, then reaching a length equal to about one-half or one-third of that of the basal rays ; but more usually the gastral ray is prolonged to a considerably greater length than the basal rays, reaching 130 /j, 140 /./. or even 150 p in length. The gastral ray then becomes excessively slender for the distal half or two-thirds of its length, and ends in a sharp point; it is not bent oralwards as Haeckel describes it, but it is either quite straight or irregularly curved. Haeckel's figure of a quadri-radiate (Kalkschwamme, iii. pi. 14. fig. 6 c) obviously represents a spicule of L. complicata (compare his fig. 1 e on pi. 15, I.e.). Quadriradiates are also to be found in which, with gastral rays of great length, are found basal rays much shorter than usual text-fig. 2, 1 g ; text-fig. 4, 4 e) ; these are probably young forms in which the rapid growth of the gastral ray * has caused it to attain its full length before the basal rays have done so. In the thick quadriradiates found in many specimens, I have observed a curious point with regard to the gastral ray, when seen in the facial aspect of the spicule. When the basal system is focussed so that the bases of the rays show sharp contours, the origin of the gastral ray appears as a dark central spot roughly triangular in outline, each side of the triangle being transverse to the base of one of the rays of the triradiate system, and the angles of the triangle rounded off (text-fig. 3, 2 a, 2 b). If now the focus is slightly raised, the base of the gastral ray appears as a sharp ring, within the triangle. The dark triangle appears to be the expanded base of the gastral ray, but it is only to be seen in the case of the thickened triradiate systems, not in the slender ones. The monaxon spicules of Clathrina contorta vary in the most singular manner, constituting the most remarkable feature of the species. The variations are best considered, first, from the point * As I have described in a former memoir (Quart. Journ. Micr. Sci., n. s. xl. pi. 42. fig. 55), the elongated gastral rays of contorta are covered by a plasmodial mass containing four nuclei, more than I liave observed on the gastral rays of anv other Ascon. |
