and Physiology of the Spongida. 29 



surface in what has been called the "vent" or "osculum, 



'? 



which varies in size, but for the most part is large and con- 

 spicuous. In what way the ends of the radicles communi- 

 cate with ampullaceous sacs in the' siliceous sponges has not 

 been satisfactorily explained ; but from their opening out of 

 these sacs so directly and conspicuously in the calcareous 

 sponges, it may be inferred that this is the case also in the 

 siliceous ones. Be this as it may, the function of the system 

 is to carry off the excrementitious matter of the sponge, as 

 may be observed in the young Sjiongilla (which at first has 

 only one canal-system, and therefore only one vent) after it 

 has been fed with carmine or indigo. 



The opening in which the main trunk of the canal-system 

 terminates may be on a level with the surface, or more or less 

 raised above it by a mammiform (nipple-like) or tubular pro- 

 longation of the dermal structure entire, or of sarcode alone ; 

 but whether of dermal structure generally or the sarcode 

 alone, the opening is always provided with a sphincter, which 

 may be closed or opened as required. This is similarly situated 

 in some sponges to the sphincter of the subdermal cavity of 

 the pore-system (that is, a little below the surface), but differs 

 from the latter, of course, in not being covered by the pori- 

 ferous sarcode of the dermis. Where the prolonged vent 

 consists of sarcode alone, the opening of course is at its 

 extremity. 



Occasionally the vents appear in little groups, distinctly 

 although irregularly disposed ; sometimes they are arranged 

 in a petaloid form, and sometimes stelliformly — that is, with 

 little gutters running to them radiatingly, in the dried state, 

 which are converted into canals by the dermal sarcode during 

 life. 



They are situated on the inner aspect of the excavated or 

 tubular sponges, and always on the depending or inner side 

 of flabellated expanded forms, which, on becoming frondose 

 and sinuously plicated, cause the depending sides to vary with 

 the sinuosities — so that the vents are found in patches, some- 

 times on one, sometimes on the other side, as determined by 

 the undulation of the frondose expansion. 



Sometimes the opening of the vent is accompanied by a 

 row of spicules, arranged round the orifice so as to lean towards 

 each other in a conical form when the opening is closed, and 

 vice versa when it is dilated. 



Although the excretory canal-system is single in the em- 

 bryonal sponge, it becomes multiplied as the latter increases 

 in size ; so that the surface of a large sponge may present 

 several vents, each of which is generally the outlet of a distinct 



