38 A MONOGRAPH OF THE 



office of a defensive spiculum, but as we find no similar 

 projection of defensive spicula on the opposite side of these 

 terminal orifices, it is evident that this position is accidental 

 rather than premeditated. I could not find any of these 

 spicula on the interstitial membranes of the sponge, between 

 the lining membrane of the cloaca and the dermal mem-' 

 brane. 



The pores are minute and are best seen by direct light 

 with a power of about 160 linear. They are situated in 

 the areas of the dermal membrane, which are formed by 

 the network of equiangular triradiate spicula, by which it 

 is supported.; every area has not a pore,* but sometimes 

 one large area will have two or three, but one is the more 

 usual number. 



The triradiate spicula of the skeleton are very variable 

 in size and strength ; some of them are quite as large as 

 the triradiate bases of the large defensive spicula of the 

 cloaca, and these are placed along with the basal portions 

 of those spicula, about midway between the dermal mem- 

 brane and the parieties of the cloaca ; while the rest of the 

 skeleton spicula do not exceed in size the ordinary dimen- 

 sions of those of the whole tribe of such sponges. 



The dermal and interstitial membranes abound in small 

 acerate tension spicula, and with minute, attenuated, spicu- 

 lated triradiate ones. The spicular ray rarely exceeds in 

 length a third or a fourth of the length of one of the basal 

 radii, and the basal rays are not, as it is usually the case, 

 in the same plane, but are projected backward in an equal 

 degree, forming, as it were, a tripod support to the spicu- 

 lar ray. 



The sponge does not appear to attain a greater size than 

 about one and a half or two inches in diameter ; the speci- 

 mens from Sark and Guernsey are smaller than those from 

 Scarborough in diameter, but very much more lobular and 

 elevated. 



