782 
DE. J. S. BOWEEBANK ON THE ANATOMY 
In some species of TetJiea, where the sponge is elaborately protected by distinct 
systems of defensive spicules, the subsphero- or sphero-stellate forms are either entirely 
absent, or only represented by minute clavate or cylindro-stellate forms ; but in Tethea 
Ingalli and in Geodia carinata, where there is an almost total absence of elongate 
defensive spicula at the surface of the sponge, the acutely pointed large subsphero- 
stellate spicula are exceedingly numerous immediately beneath the dermis, and gradually 
decrease in number in an inward direction until they almost cease to exist in the deeper 
portions of the sponge. Thus their presence in such abundance near the surface of the 
animal would tend materially to check the voracity of any enemy that might attempt to 
prey upon them. In like manner we find the smooth and abundantly porous membrane 
of Tethea muricata (figs.I4&I5, Plate XXXI.) crowded with the elongo-attenuato-stellate 
form represented in Plate XXV. fig. 18, Phil. Trans. 1858 ; and a single glance at them, 
as represented in situ, will show how admirably they are calculated to defend the delicate 
tissue on which they repose from the attacks of even their most minute and insidious 
enemies. The mode of their disposition is also strongly indicative of their defenshe 
functions, their long axis being, not parallel to the plane of the membrane beneath, but 
at right angles to it. 
In Tethea Norvegica, Boweebank, MS., where the surface of the sponge is well 
provided with external defensive spicula, the large subsphero-stellate form is compara- 
tively rare, but the tissues of the neighbourhood of the intermarginal spaces and canals 
are crowded with the minute attenuato-stellate forms, and their surfaces are bristling 
with the sharp points of their radii, so that no intruding annelid could either take a 
mouthful from their surfaces or crawl over them with impunity. Deeper in the sponge, 
beyond the range of penetration of such enemies, they are comparatively very few in 
number, and the large subsphero-stellate ones are entirely absent. 
The hexradiate forms represented by figs. 24 to 36, Plate XXV. Phil. Trans. 1858, 
are more especially found in the siliceo-fibrous sponges. I have only seen two speci- 
mens of this class of sponges in which the sarcode was well preserved. In one of these 
I have observed the slender form like that of fig. 34, Plate XXV., occupying the areas 
of the rigid siliceous skeleton completely surrounded by sarcode, which stretched from 
one ray to another in thick glutinoid plates, but without touching the surrounding ske- 
leton-fibres, excepting at one basal point connecting it with the general mass of the sar- 
codous tissues. From the positions and general appearances of the hexradiate spicula, it 
would appear that this form of spiculum has the office more especially of supporting and 
consolidating the sarcode, and that it is in no respect subservient to defensive purposes. 
Generally speaking the slender rectangulated hexradiate spicula occur singly, but I 
have sometimes found them grouped together ; in this case their axes were coincident, 
and their radii in the same plane, or very nearly so, but not always agreeing in their 
direction ; such a framework would form a very fitting support to a large mass of sar- 
codous tissue partially separated from the framework of the skeleton and occupying a 
portion of a large interstitial space. 
