OF THE SPONGIADiE. 101 



dria panicea, and Fig. 299, Plate XIX, a similar section of 

 a branch of Chalina simulans, Bowerbank, showing that, 

 however varied the forms of the sponge may be, the inter- 

 stitial cavities are the same in structure and position. 



I have never been able, in the HaHchondroid sponges, to 

 detect valvular diaphragms separating these spaces from 

 the interstitial canals and cavities beneath. 



In the genera Geodia and Vachymatisma 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, 

 Bowerbank, MS., a highly organized species of the genus, 

 they are found in the crustular dermis in great abundance. 

 They are in form very like a bell, the top of which has 

 been truncated. 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 

 stratum of membranes and sarcode, of about two-fifths the 

 entire thickiiess 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 surrounding ones, the condition or 

 degree of opening of no two adjacent ones being alike. In 

 the greater number of cases they were in a closed state, 

 and in this condition the membrane was filled with con- 

 centric 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 circular or 

 slightly oval ; in others it was nearly as large as the diameter 

 of the basal end of the cavity. The pursing of the centre 



