CALCAKEA. 39 



skeletou proper ; the gastral cortex is supported by the paired rays of the chiactines 

 aud subgastral triradiatcs (see Fig. 120). The dermal cortex is formed of a thick layer 

 of irregularly scattered triradiatcs (Fig. 124). 



The oscule is shown in Fig. 121. There is a ring of small quadriradiates lying 

 tangentially round the inner edge, with short pointed apical rays from which springs 

 the diaphragm. There is a thick fringe of hair oxea, outside which there is an open 

 fringe of large oxea. The oscule is thick and the flagellated chambers reach almost to 

 the edge. Near the oscule they are only in a single layer, l)ut in the middle of the 

 sponge they are ten or twelve layers thick. 



Spicules (Fig. 123). 

 Oxea. — There arc four sorts of Oxea : — 



[ii) Large projecting oxea, pointed sharply at the inner end, and very bluntly at 

 the outer end, which is thicker. Curved all along, more sharply near the 

 thicker projecting end. Length fairly constant about 700 /x, thickness 30./x 

 to 35 ju.. 



(A) Hair oxea, projecting in dense tufts, quite straight, length over 500 /x, 

 thickness about 1 /x. 



(c) Minute spined hastate oxea, scattered all tlirough the Ijody-wall. Slightly 

 bent, with the largest spines in a ring round the " set-over " of the spear- 

 head. Usual length GO /x, occasionally 150 /a, thickness 4 /x to 5^. 



((/) Small irregular oxea from the body-wall al)0ut 350 /x X 20 /x, often set-over 

 in the middle of their length. 



Triradintes. — There are two sorts of Triradiatcs :— 



((') Large alate subgastral triradiatcs. Basal ray straight, tapering uniformly 

 to a sharp point, about 700 /x long x IGju. thick. Paired rays equal, nearly 

 straight, 200 /n, to 320 /x long X 20 /x thick; oral angle 1G0°. 



(/") Smaller alate triradiatcs from the dermal cortex (prol)ably the same as the 

 rare ones in the body-wall). These vary widely in size. Basal ray 

 straight, tapering uniformly to a sharp point, 170 /x to 700 /x long x 10/x 

 to 18 /x thick. Paired rays equal, forming a rounded bend at the centre, 

 but straight for the rest of their length, 120 /x to 270 /x long x 10/x to IG/x 

 thick. Oral angle very constant, 108° to 112°. 



(//) Chiactines. Basal ray straight, sharply pointed, GOOyix to 750 /x long x 14/x 

 thick. Paired rays bent more or less forwards, 200 /x to 280 /x long x 16 /x to 

 20 /x thick. Oral angle varial)lc, 135° to 160°. Apical ray about 80 /x X 12 /x. 



Oscular spicules. 



{h) Large oxea like (a), but shorter ; 380 /x to 480 /x long X 18 /m thick. These 

 form an external fringe. The inner fringe is formed of hair oxea of 



unknown length. 



N 



