18/2.] DR. J. S. BOWERBANK ON SPONGIADjE. 199 



the proximal ends of many of them scarcely passing through the 

 dermal crust of the sponge. On the more exposed parts of the 

 sponge only a very few were to he found ; and, what is very remark- 

 able, some of the deepest and most protected depressions were 

 equally destitute of them. 



In the first specimen, when the two parts into which it was divided 

 were put together, there were the remains of two very large depressed 

 areas, and one smaller but perfect one ; the latter was nearly circular, 

 with gradually rounded edge, and at the level of the outer surface 

 was 1| inch in diameter, and f inch deep. The bottom of this area 

 was crowded with small oscula, none of which exceeded half a line 

 in diameter. Remains of the same cribriform arrangement of the 

 oscula was apparent in the portions of the larger areas ; and in these 

 some of the oscula were as much as a line in diameter. In the 

 second specimen there is but one, large, nearly circular, depressed area, 

 which decreased in size from 1 inch at the surface-level to f inch at 

 its smallest diameter ; half an inch within the level of the surface it 

 expanded into a great oval cavity 2 inches deep from the surface- 

 level ; and the whole of the interior, from immediately within the 

 level of the greatest contraction of the orifice, was lined with closely 

 packed oscula, presenting the same cribriform arrangement as in the 

 specimen first described. Nearly all of the oscula in both specimens 

 were closed by a stout membranous veil. When a portion of these 

 oscula were immersed in water and examined by transmitted light 

 with a power of 260 linear the membranous veils were seen to be 

 depressed below the level of the surrouuding margin, faint concen- 

 tric lines or ridges were apparent, and a thickening and pursing of 

 the membrane near the middle of the area was visible. From the 

 marginal ring of the oscula numerous small fusiformi-acerate dermal 

 spicula were projected for about a fourth or a third of their length ; 

 and deeper within the margin a few were projected for at least three 

 fourths of their length : this projection of spicula is apparently to 

 prevent the intrusion of vermes or other predaceous animals ; and 

 both by structure and position they are admirably adapted for such 

 a purpose. 



The depressions immediately above the intermarginal cavities, 

 which appear like pores in the dried specimen, are barely visible 

 with an inch lens ; and in a portion of the first specimen described, 

 which was preserved in salt and water immediately on being taken 

 from the sea, they were not in the slightest degree visible with the 

 same power. 



The dermal membrane covers the external expanded orifices of 

 the intermarginal cavities. It is perforated by numerous minute 

 pores equally dispersed over the surface, and which apparently have 

 the power of opening or closing at the will of the animal. When a 

 portion of the crustular dermis was cleared from the connecting 

 spicula, so as to expose both the outer and inner surfaces to view, 

 and then mounted in Canada balsam, many of the pores were found 

 in an open condition ; but in the specimen preserved in salt and water 

 none could be detected in that state. 



