THE SPONGIDA OR PORIFERA. 347 



he said : — " The fgecal apertures are raised to the extremities of 

 projecting papillae, in such sponges as cover the sides of rocks, in 

 order to convey the excrement beyond the pores and general 

 surface of the animal. In such branched species as have a soft, 

 downy surface, the faecal orifices are ranged in close order along 

 the outer margins of the branches, and very few are observed on 

 the fiat 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 require, projecting papillae, because they hang sus- 

 pended 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 conipressa 

 (Fig. 4, Plate XIV.), which always hangs down and whose orifices 

 are always marginal. The bright yellow, porous, placentiform 

 mass of the Spongia panicea ( H. encnistans, Fig. 3, Plate XIV.) 

 has no papillae ; indeed, the faecal orifices are sometimes even lower 

 than the general surface of the animal, and I have never seen this 

 sponge, excepting on the under surface of rocks, with its orifices 

 perpendicularly downwards, so that the excrements fall clear of its 

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

 The flat species which are found encrusting Fuci, Sertularia, 

 Corallines, or other moveable bodies, have very seldom prominent 

 papillae, because they are cleansed by the agitation of the sea like 

 the branched sponges." 



Grant not only proved that the currents from the interior of 

 the Spongida were discharged through the oscula and larger ori- 

 fices ; but he also showed that the surrounding water passed into 

 these animals through the innumerable \\ny pores. 



" I first placed," said Grant, "a thin layer from the surface of 

 the L. papillaris in a watch-glass with sea-water under the micro- 

 scope, and on looking at its pores I perceived the floating particles 

 driven with impetuosity through these openings ; they floated with 

 a gentle motion to the margin of the pores, rushed through with a 

 greatly increased velocity, often striking on the gelatinous network, 

 and again slackened their course when they had passed through 

 the openings. The motions were exactly such as we should expect 



