OF THE SPONGIAD^. 41 



sponges of commerce, the honeycomb sponge of dealers, in 

 which siliceous spicula play a very subordinate part in the 

 construction of the skeleton. 



The tricurvo-acerate form in all its varieties is better 

 calculated to effect their peculiar office in small and irregu- 

 lar spaces, and with greater economy in numbers, than the 

 straight elongated forms, and they are also better adapted 

 to membranes having unequal surfaces, such as those in 

 Microciona armata, Bowerbank, where we see them follow- 

 ing the undulations of the membranes and sustaining them 

 in their proper positions around the columnar parts of the 

 skeletons. The varieties of form in these spicula are well 

 represented in Figs. 96, 97, and 98, Plate IV. They are 

 all out of the same sponge. In Grantia compressa, and 

 other closely allied species, where the structure is systema- 

 tically membranous, the skeleton spicula are triradiate, 

 supporting the membranes in uniform planes in the most 

 effectual manner ; and they are in fact systematically tension 

 spicula, as well as skeleton ones. In Lenconia nivea, Bower- 

 bank, which is not symmetrical in its structure, like G. com- 

 pressa and its congeners, other forms of tension spicula are 

 developed to suit their especial purposes, such as repre- 

 sented by Figs. 100 and 101, Plate IV. 



In siliceous sponges we also occasionally find triradiate 

 spicula developed and performing the office of tension 

 spicula in the midst of comparatively large membranous 

 areas ; but these forms, in every case under such circum- 

 stances in which I have seen them in situ, appear to belong 

 to the exception, rather than the general rule obtaining in 

 such sponges. 



The foliato-peltate spicula for a full account of the pro- 

 gressive development of which I must refer to Terminology, 

 number 102 appear to be a development of the apices of 

 connecting spicula into dermal tension ones, bearing a 

 strong resemblance in form and purpose to the bony scutes 

 in the skins of some of the higher animals ; while the ex- 

 treme crenulation of their margins probably served the 

 purpose of facilitating the action of the porous system. 



In all the varieties in form which I have hitherto de- 



