Siliceous Spicules of Sjxmges. 211 



the axis and arms being long, slender, and subulate, they are 

 short, thick, and truncated at the end, and are more or less 

 covered with spicules (see Schultze, Hyalonema, t. iii. f. 14, 15). 

 Dr. Bowerbank gives five figures of the spicules (all in a 

 more or less imperfect state) of the sponge from this situation. 

 He does not figure one in its perfect state, with all the six 

 arms' developed ; but he figures one with one end of the axis 

 wanting and the rest present (B. S. t. vi. f. 157). Then he 

 figures other cross-shaped spicules with both ends of the axis 

 wanting (B. S. t. vi. f. 154-156), and one in which the un- 

 branched axis alone is developed (B. S. t. vi. f. 153). 



O. Schmidt figures several modifications and adhesions of spi- 

 cules of this type in his * Spongienf.' t. i. & ii., showing many 

 peculiarities. 



When all the lateral arms in this kind of spicule are want- 

 ing, the axis, which is simple and needle-like, is known from 

 the simple spicule by having a well-marked cross in the minute 

 belt in the centre of the spicule (B. S. fig. 65). 



Schmidt figures a spicule of this type in which the axis and 

 lateral arms are of equal length, all tapering and acute, and 

 with irregular spines on the side (Spongienf. t. i. f. 5), so that 

 it might almost be taken for a stellate spicule. 



When the sexradiate spicules are placed on the surface of 

 the sponge, as they often are, forming a kind of armour to it, 

 with the inner axes of the spicules diverging from the centre 

 and the lateral arms forming the outer surface of the sponge, the 

 outer part of the axis (which would otherwise project beyond 

 the sponge) is more or less completely wanting. This form of 

 spicule is called " the nail-like spicule " (see Schultze, Hyalo- 

 nema, t. iv. f. 5-9). See Carter, Ann. & Mag. Nat. Hist. 

 1869, iv. t. i. & ii. ; 1871, vii. t. x. & xvii. ; 1872, ix. t. x., 

 t. xxi. & xxii. ; 1872, ix. t. xx. 



When these sexradiate spicules arise from the outer surface at 

 the base of a sponge, and are used as anchors to keep the free 

 sponge in its place, the axes of the spicules are often very long 

 and hair-like, and have four short, recurved, broad rays at 

 the end, the axis not extending beyond the rays (B. S. t. iii. 

 f. 59). Bowerbank calls these " apically spined recurvo- 

 quaternate prehensile spicula." Probably the spicule figured 

 in B. S. t. iii. f. 73-76 as " spinulo-recurvo-quaternate spicula" 

 is also a sexradiate spicule; but I have not seen it. The 

 spicule figured B. S. t. xviii. f. 292 also belongs to this 

 modification of the sexradiate spicule. These spicules are 

 not to be confounded with the " fusiformi-recurvo-temate spi- 

 cula" (B. S. t. iv. f. 81, 82) with three terminal rays, which 

 evidently belong to the quinqueradiate spicula. Mr. Carter 



