HYALONEMA (HYALONEMA) OBTUSUM. 157 



Strands of longitudinal rhabds enter the waU of the gastral cavity and the 

 gastral cone from below. The rhabds of the gastral wall for the most part 

 follow the gastral membrane, and here form a dense and distinct subgastral 

 layer. The rhabds of the cone lie partly superficially, partly axially. The 

 superficial rhabds of the cone are more slender than the axial ones. The axial 

 rhabds are congregated in strands which together form a loose column extending 

 quite to the summit of the cone (Plate 34, fig. 3). A few hexactine megascleres 

 apparently with long longitudinal and shorter transverse rays also take part in 

 the formation of this column. The micramphidiscs not only form a dense layer 

 on the outer surface of the gastral cone, but also extend for some distance into 

 its interior. Farther down, at a level about 0.3 mm. below the surface, micro- 

 hexactines and microhexactine-derivates, similar to those of the choanosome, 

 make their appearance in the cone. These spicules extend down to a depth of 

 about 0.8 mm., thus occupying a zone about 0.5 mm. thick. The inicrohexac- 

 tines are very numerous in this zone, the microhexactine-derivates rare. The 

 central part of the cone, in which the axial column of longitudinal rhabds ex- 

 tends, is destitute of microscleres. 



The skeleton of var. robusta appears to be on the whole similar. The micro- 

 hexactine-derivates are more various ; the uncinates attain a larger size, and reach 

 down to greater depths of the sponge. The upper ends of the large stalk-spicules 

 are still present, and the felt formed by the slender acanthophores in the basal 

 part of the sponge-body is denser and more extensive (Plate 39, figs. 22-24). 



The dermal pinules (Plate 35, figs. 23, 24a, 25, 29-37; Plate 40, figs. 4, 5) 

 are mostly pentactine, but hexactine forms also occur, differing from the pentac- 

 tine ones only bj^ possessing a sixth, proximal, apical ray. The distal apical ray 

 is generally straight, rarely angularly bent below the middle of its length (Plate 

 35, fig. 25). In the dermal pinules of var. gracilis 137-165 n, the ray is usually 

 143-154 /i long and 4-5 yu thick at the base. Farther up it thickens and it gener- 

 ally terminates with a stout, broad, and blunt cone 8-11 n tliick. Rarely it is 

 terminally rounded, dome-shaped, and has a maximum thickness of 12 yu (Plate 

 35, fig. 31). The proximal part and the terminal cone (or dome) of the distal 

 ray are smooth, its middle-part bears sharp, conic spines. The lowest of these 

 spines are sparse, short, and strongly divergent. Farther up they become more 

 numerous. The size of the spines increases up to the middle of the length of the 

 distal ray and then again decreases; the inclination of the spines towards the 

 tip of the ray increases continuously quite to the end. The largest spines attain 

 a length of 13-19 n, and are 2-4 n thick at the base, usually about 3.5 yu. The 



