HYALONEMA (HYALONEMA) AGASSIZI. 
175 
be similar to that of Hyalonema ( Hyalonema ) obtusum var. gracilis; the chief 
difference apparently being that the former is more dense and has narrower 
subdermal cavities and canals. The flagellate chambers are elongate and 50-80 m 
broad. In one of the specimens of form C the afferent canals are very clearly 
visible. They here appear as tubes, about 0.5 mm. wide, which lie parallel side 
by side and extend vertically down into the interior of the choanosome. In 
this form, and in the forms B, D, and E, the gastral cavity is divided by radial 
vertical plates into diverticula. The plates are, in several of these specimens, 
four in number and regularly arranged in a cruciate manner. The diverticula 
extend downward, are tubular, very wide above, attenuated below, and nearly 
circular in transverse section. Their walls are perforated by numerous efferent 
apertures, many of which attain considerable dimensions. 
Skeleton. The whole of the outer surface of form A, and the (small) parts of 
it, in the other forms where the dermal membrane is still present, are covered 
by a dense pinule-fur (Plate 42 , fig. 36; Plate 45 , fig. 23a). Certainly in form A 
and probably also in the other forms, the pinnies of all parts of the outer surface 
are similar, with the exception of the part close to the origin of the stalk. They 
are in all forms for the most part pentactines; a few, however, possess a more 
or less developed sixth, proximal ray, and appear as hexactines. Between the 
lateral rays of these dermal pinules a few micramphi discs lie scattered on the 
outer surface. From the thin, upper, free margin of the wall surrounding the 
gastral cavity (fissure or depression) the distal rays of cliactine pinules protrude. 
The gastral surface, that is the inner surface of the wall surrounding the gastral 
cavity, and the surface of the gastral cone are likewise covered by a pinule-fur. 
The pinules composing it are chiefly pentactines, more rarely hexactines, excep- 
tionally diactines. A few minute spiny pentactines have also been observed 
here. On these gastral surfaces also micramphidiscs occur. These spicules are 
here, however, much more abundant than on the outer surface, and in places 
form dense masses. Below, where the gastral cavity passes into the large effer- 
ent canal-stems, the pinule-fur ends; the coating of micramphidiscs, however, is 
continued along the walls of these canals quite down to the innermost parts of the 
choanosome. The micramphidiscs of the outer, dermal surface and of the 
surfaces bordering on the upper part of the gastral cavity and enclosing the inner, 
proximal parts of the efferent canals, are all or nearly all small ones. Those on 
the surfaces surrounding the lower proximal part of the gastral cavity and the 
mouths of the large efferent canals on the other hand are, certainly in form A, 
and probably also in the other forms, in great part large macramphidiscs. 
