274 J. G. WALLER ON THE BURROWING SPONGE. 



it seeks the more complete protection offered by burrows or 

 hollows, vacant spaces having more secure walls. Another very 

 interesting sponge, which I found abundantly on the same oyster- 

 shell which has given me so much information on this " boring 

 sponge," does, however, build within crevices and parts of the sand- 

 built tubes just alluded to. 



I will now turn to some points of great interest which seem not 

 hitherto recorded. I have here a section (Fig. 1.) representing the 

 burrows filled with the sponge. At its distal extremity, as I have 

 already mentioned, where is the latest growth of the sponge, we find 

 the membrane pellucid, with but few spicula. Going back into the 

 older portions, this membrane gets gradually more full of colour, 

 denser in character, the spicula increasing in number, until they 

 become almost matted together. We then come to that part on 

 the edge of the shell where the laminae are wide and open, whose 

 spaces, beside the burrows and connecting with them, the sponge 

 has occupied, and here has communication with the outer world. 

 Hitherto the dermal membrane has been protected by the shell it 

 covers ; now, as this is wanting, we find the means it takes for 

 that purpose singularly effective. The spicula in all other parts of 

 the sponge have been irregularly disposed on the membranes, which 

 is a mark of the genus to which Dr. Bowerbank has given the 

 characteristic name of Hymeniacidon. But here they form them- 

 selves into a regular wall, closely packed together, parallel to each 

 other, so as to present a formidable array of pikes to any adven- 

 turous intruder who would attack the domicile. (Fig. 2.) The 

 sarcode here thickly overlays and invests it. Here are found the 

 pores — the inhalent organs — not, however, very conspicuous, but 

 sometimes distnctly seen with an inch lens. So that, instead 

 of finding the sponge in possession of an apparatus of attack and 

 destruction, which has been assigned to it, we see that its offensive 

 powers, if we may so call them, have been concentrated into a system 

 of defence, to screen it from assaults from without; and that no part 

 of the sponge has so powerful an organizaton as this prepared for 

 its protection. 



It is natural now that we should pass to consider those organs 

 by which the excretory function is performed, viz., the oscula. The 

 excretory organs of a sponge are always remarkable, whether they 

 consist of simple oscula, or whether of a number of oscula 

 pouring their contents into a common sewer or cloaca. In many 



