124 BRITISH SPONGES: 



strongest currents, and which is therefore best suited for the 

 observation of the peculiar circulation of the family. Its fecal 

 orifices are not raised on papillary eminences, because, says Dr 

 Grant, the sponge grows always on the under side of rocks look- 

 ing down in the water, and hence an elevated crater, to throw 

 off from the surface the excrementitious matters which are eject- 

 ed from it, is unnecessary. " The fecal apertures are raised to 

 the extremities of projecting papillae, in such sponges as cover 

 the sides of rocks, in order to convey the excrements beyond the 

 pores and general surface of the animal. In the Spongia ocu- 

 lata, S. palmata, S. xerampelina, and such branched species as 

 have a soft downy surface, the fecal orifices are ranged in close 

 order along the outer margins of the branches, and very few are 

 observed on the flat surface, in order to prevent the excrement 

 from falling in the direction of the flat woolly surfaces, which 

 would be very apt to retain it, and thus choke up the groups of 

 pores which are seen everywhere over their surface. Such 

 branched sponges have not, and do not reqmre, projecting pa- 

 pillae, because they hang suspended by a narrow stem, and are 

 kept sufficiently clean by receiving gentle undulations from the 

 constant motions of the sea. The same applies to the soft 

 downy white Spongia compressa, which always hangs dowTi, and 

 whose orifices are always marginal. The bright yellow porous 

 placentiform mass of the Spongia panicea has no papillae ; in- 

 deed, the fecal orifices are sometimes even lower than the gene- 

 ral surface of the animal, and I have never seen this sponge ex- 

 cepting on the under surface of rocks, with its orifices perpendi- 

 cularly downwards ; so that the excrements fall clear of its sur- 

 face by their own gravity, without the assistance of papillae. 

 The flat species which are found encrusting Fuci, Sertulariae, 

 Corallines, or other moveable bodies, have very seldom promi- 

 nent papillae, because they are cleansed by the agitations of the 

 sea like the branched sponges."' Grant. 



