124 Prof. W. Thomson on the “ Vitreous” Sponges. 
_ Another set from Aphrocallistes are especially interesting 
(woodcut, a): they consist of a lengthened shaft ending in 
a small expansion, from which spring four equal branches, 
each terminated by a little knob. No doubt these are the 
separated branches of a complex hexradiate spicule closely re- 
sembling those figured by Bowerbank (British Sponges, vol. i. 
figs. 190-192) from species of the penultimate genus of our 
series, Dactylocalyx. 
General Structure of the Sponges. 
The netted walls of the two species of Habrodictyon are 
formed of a loose open network of fascicles of comparatively 
short fusiform spicules, the fascicles loosely bound together by 
the soft sarcode. The bundles curve irregularly in all direc- 
tions, so that the network is quite irregular. No distinct 
bundles of long fibres pass longitudinally from end to end of 
the sponge, or transversely round it as in Huplectella. In H. 
corbicula the upper truncated end of the tube is closed by a 
netted lid, of a denser aggregation of spicules than that which 
forms the general wall; while in /. speciosum the general wall 
extends uniformly over the enlarged end of the tube without 
any change of structure. 
The spongy portion at the base of Hyalonema resem- 
bles closely in minute structure the wall of Habrodictyon. 
The spicules are nearly of the same form, and are arranged 
much in the same way ; but the bundles of needles are meshed 
into a porous conical mass whose parts tend to radiate towards, 
to be combined with, and to support a bundle of enormous 
spicules, whose lower portion is twisted into a close, compact, 
tapering coil in the centre of the sponge, while the upper part 
of the coil projects a foot above the centre of the sponge, and 
is frayed out in the water like a glittering brush of glass*. 
In Euplectella, the long vertical spicules, instead of formin 
an isolated wisp as in Hyalonema, are separated into smal 
fascicles, which are spread out symmetrically and connected 
into a netted tube by ring-like bundles of transverse fibres. 
The result is a wonderfully beautiful and symmetrical net 
with square meshes. Over this framework the general sponge- 
* I have not space at present to discuss the curious diversity of opinion 
which exists as to the relations of Hyalonema. I will only refer the reader 
to an admirable memoir by Professor Max Schultze, ‘ Die Hyalonemen,’ 
Bonn, 1860, to a second pepe the same author in the ‘Annals and 
Magazine of Natural History,’ March 1867, and to a paper of my own in ~ 
the ‘ Intellectual Observer’ of the same date. 
