HOLASCUS EDWARDSII. 23 



oxyhexasters very scarce. The graphiocomes are also rather rare and nearly 

 always destitute of end-rays. The ring-shaped sigms are very numerous, both 

 in the centrifuge spicule-preparations and in the sections, but in spite of this I 

 am not at all sure that they are proper spicules of the sponge. They may, 

 Uke the masses of other siliceous skeletal structures found in the sponge, be 

 altogether foreign to it. 



Below the outer surface of the tube-wall hypodermal hexactines with two 

 radially situated, differentiated rays occur. The distal, differentiated, some- 

 what protruding ray is short, stout, and spined. It raises the dermal membrane 

 conule-fashion. The four not differentiated (lateral) rays extend paratangen- 

 tially. The proximal ray is elongated. To the distal rays of these hexactines 

 slender, simple or centrotyle, comital diactines are attached, which, when 

 numerous, form a sort of mantle around it. Below the inner surface similar, 

 hypogastral, hexactines are situated. The distal rays of these spicules are, how- 

 ever, more slender and destitute of comitals. 



The root-tuft consists chiefly of very long diactine anchor-spicules. A few 

 spined styles and tylostyles, with the blunt end situated distally, are also found 

 in it; these may, however, be foreign to the sponge. 



The choanosomal centrotyle rhabds (Plate 19, figs. 22-24) are 290 ii~l.7 mm. 

 long and, near the middle, 5-47 n thick. The small ones, under 400 ^ in length, 

 are fairly numerous, the larger ones rare. The tyle is usually more (Plate 19, 

 fig. 23) or less (Plate 19, fig. 22) toward one end, more rarely situated centrally 

 (Plate 19, fig. 24). It consists of four ray-rudiments, which are, however, 

 so small in some, particularly the large centrotyle rhabds, that they can hardly 

 be individually distinguished. The tyle measures 14-60 n in transverse diame- 

 ter. In the small centrotyle rhabds it is relatively large, the proportion between 

 the tyle-diameter and the thickness of the adjacent parts of the spicule being 

 here 2:1 to 3.5 : 1. In the large centrotyle rhabds the tyle is relatively small, 

 this proportion being here 1.27: 1 to 1.5: 1. The two rays are conic or cylindric 

 and sharp-pointed or, more frequently, blunt. The largest centrotyle rhabds 

 appear to be quite smooth. The small ones are spiny, particularly near their 

 ends. The degree of spinulation is on the whole in inverse proportion to the 

 length of the rhabd. 



The slender, thread-like rhabds observed in the small specimen are under 1 n 

 thick and relatively very long. 



The rhabd cornitals of the distal rays of the hypodermal pentactines are straight 

 or slightly curved, simple or centrotyle, and generally 200-400 yu long and 2-2.5 n 

 thick. 



