788 
DE. J. S. BOWEEBANK ON THE ANATOMY 
Plate XXXII. a similar section of a branch of Halichondria dmulmis, Jokn'STOX, showing 
that, however varied the forms of the sponge may be, the interstitial cavities are the 
same in structure and position. 
I have never been able, in the Halichondroid sponges, to detect valvular diaphragms 
separating these spaces from the interstitial canals and cavities beneath. 
In the genera Geodia and Pachymatisma these organs assume a very much greater 
degree of regularity and a complexity in their organization that are never apparent in 
those of the Halichondroid sponges. In Geodia Barretti, Boweebam:, MS., a highly 
organized species of the genus, they are found in the crustular dermis in great abun- 
dance. They are in form very like a bell, the top of which has been trrmcated. They 
are situated in the inner portion of the dermal crust ; the large end of the cavity being 
the distal, and the smaller end the proximal one. The open mouth or distal end of the 
cavity is not immediately beneath the dermal membrane. There is an intervening stra- 
tum of membranes and sarcode, of about two-fifths the enthe thickness of the dermal 
crust, which is permeated by numerous minute canals which convey the water inhaled 
by the pores to the expanded distal extremity of the cavity. The proximal end is closed 
by a stout membranous valvular diaphragm, which the animal has the power of opening 
and closing at its pleasure. It is usually entirely destitute of the characteristic dermal 
spicula that are found abundantly in the adjoining membranous tissues. 
The action of the diaphragm of each cavity appears to be independent of the surround- 
ing ones, the condition or degree of opening of no two adjacent ones being ahke. In 
the greater number of cases they were in a closed state, and in this condition the mem- 
brane was filled with concentric circles composed of minute rugae or thickened lines, and 
at the centre it was closely pressed together, completely closing the orifice. In some 
cases the membrane was only partially closed, and the orifice was either chcular or 
shghtly oval ; in others it was nearly as large as the diameter of the basal end of the 
cavity. The pursing of the centre of the membrane of the diaphragm was always out- 
ward as regards the cavity, so that when viewed from withui it appeared as a slightly 
funnel-shaped depression, the bottom of which was conical. The cavities are lined by 
a smooth and tolerably strong membrane, abundantly supplied with slender fibrous 
tissue, disposed in nearly parallel lines at right angles to the long axis of the cavity. 
The adaptation of the skeleton to the support of these elaborately constructed organs 
is very remarkable. The sponge is furnished abundantly with large expando-ternate 
spicula, the radii of which are furcated at their apices. They occur in a series of 
bundles ; the long attenuated shafts of each fasciculus approximate at their bases, and 
diverge thence until the ternate head of each is about equally distant from its surround- 
ing neighbours, and the extremities of the rays touch or slightly cross each other, thus 
forming a beautiful and regular network, the meshes being six- or seven-sided, according 
to circumstances. The upper surfaces of the radii are firmly attached to or partially 
imbedded in the under surface of the crustular stratum, and the areas thus formed are 
occupied each with the proximal valvular terminations of one of the intermarginal cavities. 
