DENDY— HOMOSCLEROPHORA AND ASTROTETRAXONIDA 255 



collection the latter explanation is probably correct. The specimen measured about 

 25 mm. in length by 8 mm. in breadth and 3 mm. in thickness. The colour on the 

 surface is almost white, internally more yellowish. The surface is almost smooth, very 

 minutely hispid. A group of some 20 or 30 minute, inconspicuous openings, situate close 

 to the margin, but not in any special depression, represent the vents. 



Paraffin sections show that each of the small vents is really covered in by a thin 

 pore-sieve, which roofs over a funnel-shaped chone that penetrates the cortex. The 

 inhalant pores are very difficult to make out, but I conclude from the study of sections 

 that they are scattered over the surface, probably in groups, and lead into narrow canals 

 which penetrate the cortex, anastomosing as they go. The cortex is about 0'7 mm. thick, 

 densely charged with sterrasters and with a superficial layer of minute spherasters. It 

 contains a good deal of fibrous tissue connecting the sterrasters with one another and 

 especially strongly developed round the inner ends of the exhalant chones, where it 

 apparently forms sphincters. 



The skeleton in the deeper part of the sponge is confused, but loose bundles of 

 diactinal megascleres radiate towards the surface, terminating distally in tufts of triasnes 

 which penetrate the cortex. The stout orthotrigenes for the most part extend their cladi 

 in the outermost part of the cortex, just beneath the surface ; beyond them again loose 

 tufts of slender spicules project freely and are almost invariably broken off short ; some of 

 the latter are very slender oxea (?styli), others are reduced anatrisenes with vestigial 

 cladome. 



Spicules : — (l) Orthotrieenes (Plate 47, fig. 3 a); with long, straight or slightly curved 

 shaft, gradually and finely pointed at the end or somewhat blunted, measuring about 17 

 by 0'034 mm. ; with short, stout, sharp-pointed cladi extended almost at right angles to 

 the shaft, measuring about 0'086 by 0'034 mm. 



(2) Anatrisenes (fig. 3 b) ; with long, slender, almost hair-like shaft, tapering away 

 to a very fine point, and sharply recurved cladi, rather long, slender and gradually and 

 finely pointed. Shaft about 17 by 0-008 mm. ; cladi about 0-07 by 0-006 mm. The 

 cladome may be reduced to a mere knob. 



(3) Oxea (fig. 3 c) ; long, slender, sometimes slightly curved ; may be sharply pointed 

 at each end, but more often one or both ends is more or less rounded off, giving rise to 

 stylote (fig. 3 c') and strongylote (fig. 3 c") forms ; measuring about 1-4 by 0-023 mm. The 

 long, very slender oxea (? styli) projecting from the dermal surface may be regarded as 

 modifications of these. 



(4) Sterrasters (figs. 3 d- — 3 d""); of typical oval form, with distinct hilum and surface 

 reticulation formed by the stellate ends of the fused rays ; size about 0'123 by 0-1 mm., 

 but many smaller ones occur which appear to be fully developed. Developmental stages 

 are shown in figs. 3d' — 3d"". 



(5) Spherasters (fig. 3 e) ; with stout, smooth, conical but somewhat irregular rays, 

 resembling an Aurora or Donatia spicule ; total diameter about 0-05 mm. Choanosomal 

 (subcortical). 



(6) Spherasters (fig. 3/) ; with relatively longer, more slender and minutely spined, 

 conical rays; total diameter about 0-037 mm. Choanosomal. 



