HYALONEMA (HYALONEMA) OBTUSUM. 155 



irregularly situated. The small macramphidiscs are also numerous and often 

 arranged in groups (Plate 34, figs. 1, 19b; Plate 35, fig. 24b). Their shafts ex- 

 tend radially or obUquely and their distal parts protrude freely beyond the 

 surface. 



The dermal membrane is supported by hypodermal pentactines very 

 variable m size. In the upper parts of the sponge the large pentactines greatly 

 predominate, at the base the small ones are more numerous. The centres of the 

 large hypodermal pentactines are about 0.7 mm. apart. The apical rays of 

 these spicules are directed radially inward (Plate 34, fig. Ic); their lateral rays, 

 which are markedly incUned towards the apical ray, extend nearly paratangen- 

 tially in the beams of the superficial network above referred to. Uncinate 

 amphioxes, situated for the most part radially or obliquely, are met with in the 

 subdermal region. The superficial part of the choanosome underlying the 

 dermal surface is occupied, down to a depth of about 2.5 nmi., by hexactine 

 megascleres, rather regularly arranged in several paratangentially extending 

 layers. These hexactines are situated so that two of their rays extend radially 

 (inwards and outwards) , two longitudinally (upwards and downwards) , and two 

 transversely (to the right and left). The distance between the centres of these 

 spicules is less than the length of their rays, and the opposite rays of adjacent 

 ones usually extend for some distance side by side and close together (Plate 34, 

 fig. 19d). These hexactine megascleres, therefore, form a three-dimensional 

 network with fakly regular, somewhat cubic meshes. These spicules vary 

 greatly in size ; the larger are situated proximally, the smaller distally. 



Numerous rhabd-megascleres and a few angularly bent diactines of similar 

 dimensions occur in the choanosome. Most of the rhabds are blunt amphioxes 

 or amphistrongyles, but styles and tylostyles also occur. Some of these rhabds 

 are isolated; most of them, however, form loose strands. In the central (axial) 

 part of the choanosome, the rhabds extend for the most part longitudinally; in 

 the other parts of the choanosome they are mostly dii-ected obliquely upwards 

 and outwards, and generally lie in the walls of the canals. The styles and tylo- 

 styles are situated so that their rounded (thickened) end points downward and 

 inward, their pointed end upward and outward. The choanosome is rich in 

 microscleres. Large numbers of micramphidiscs are imbedded in the canal-walls 

 and throughout it are scattered some macramphidiscs, masses of microhexac- 

 tines (Plate 34, fig. 2), and a few microhexactine-derivates, chiefly with only 

 two opposite rays fully developed and the others more or less, sometimes entirely 

 reduced. 



