HYALONEMA (HYALONEMA) OBTUSUM. 
155 
irregularly situated. The small macrampliidiscs are also numerous and often 
arranged in groups (Plate 34 , figs. 1, 19b; Plate 35 , fig. 24b). Their shafts ex- 
tend radially or obliquely and their distal parts protrude freely beyond the 
surface. 
The dermal membrane is supported by hypodermal pentac tines very 
variable in 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. lc); their lateral rays, 
which are markedly inclined 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 mm., 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 fairly 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 directed 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. 
