17G HYALONEMA (HYALONEMA) -AGASSIZI. 



Just below the level in which the lateral rays of the dermal pinules of the 

 outer surf?<;e extend, the paratangentially situated lateral rays of hypodermal 

 pentaetines are met (Plate 42, fig. 37a; Plate 45, fig. 23). In form C these 

 lateral pentactine rays extend in the beams of the superficial network above 

 described. The apical rays of the hypodermal pentaetines extend radially in- 

 ward. In form A a superficial zone about 0.6 mm. thick, underlying the dermal 

 membrane, is occupied by dense masses of more or less radially arranged unci- 

 nates and irregularly scattered microhexactines and microhexactine-derivates 

 (Plate 45, fig. 23). This zone contains no spicules besides these and the proxi- 

 mal rays of the hypodermal pentaetines, which traverse it. Below this zone 

 hexactine megascleres begin to make their appearance. Those lying nearest 

 the surface are quite small, towards the interior they increase in size. Though 

 often irregularly disposed in the sections, these spicules are, in the living sponge, 

 in all probability regularly arranged in such a manner that two opposite rays 

 extend longitudinally upward and downward, two radially outward and inward, 

 and two paratangentially and laterally to the right and left. In most of the large 

 and in a good many of the smaller hexactines the two opposite longitudinally 

 extending rays are longer than the other four. Masses of large macramphidiscs 

 are met with a little below the level where the hexactines begin to make their 

 appearance. In some places these form but a thin layer, in others they extend 

 a considerable distance, 2 mm. or more, into the interior of the choanosome. 



The inner parts of the sponge are occupied by the large hexactine mega- 

 scleres referred to above, and also by rhabd-megascleres, uncinates, microhexac- 

 tines, microhexactine-derivates, amphidiscs, and spheres. 



The large inner hexactines usually have two opposite, longitudinally extend- 

 ing, greatly elongated rays and four shorter transverse rays. The rhabds of the 

 axial part of the sponge are situated longitudinally and form a kind of axial 

 column, which extends upwards to the summit of the gastral cone. Loose 

 strands of rhabds diverge from this axial column and extend upwards and out- 

 ward. Below, in the interior of the choanosome, these diverging rhabd-strands 

 dissolve into scattered, obliquely situated, isolated rhabds; above they join to 

 form distinct layers lying below the dermal and the gastral surfaces of the thin 

 frill-like marginal part of the gastral wall. In the forms B, C, and D masses of 

 longitudinal rhabds also occupy the vertical radial plates connecting the gastral 

 cone with the gastral wall. Most of these rhabds are very blunt amphioxes or 

 amphistrongyles ; but sharp-pointed amphioxes, amphityles, styles, and tylo- 

 styles also occur among them. In the axial column of form A both large and 



