HALICHONDRIA. 91 



into one and then into more currents, these gradually make for 

 themselves channels in the cellular texture, the fibres of which are 

 pushed aside and prevented, by the continuance of the stream, 

 from again encroaching on its course. The channels increase in 

 number with the continued increase of the sponge, and as it cannot 

 but happen that they shall occasionally open into and cross each 

 other, we have a wider canal formed by the additional flow of 

 water into it. Such of these canals as reach near the surface 

 soon effect for themselves an opening there, for the current in 

 it pushes against the superficial coat which opposes its efilux, 

 and gradually thins and loosens its texture until this ultimately 

 disappears, leaving a fecal orifice or osculum. This is fre- 

 quently a simple circular hole ; but often, on looking within the 

 outer rim, we notice in the funnel from two to five lesser oscu- 

 la united together, which are the openings of so many canals 

 that have united there ; and sometimes we find spread within 

 the osculum, or over its mouth, a net-work of finer texture than 

 the rest of the sponge, but otherwise of the same nature and 

 composition. 



Such we believe to be the manner in which the canals and 

 oscula are formed, and hence we cannot give our assent to the 

 notion that the net-work spread over or within them is intended 

 as a " wise provision" against the intrusion of noxious animals 

 or other foreign bodies within the sponge, which seems indeed 

 to be sufficiently protected at these orifices by the efflux of the 

 currents passing continually from them. Neither can it be sup- 

 posed that the position and elevation of the oscula have any 

 foreseen relation with the situation of the sponge in the water. 

 When, according to Dr Grant, this production spreads level on 

 a rock with an upright aspect, the oscula are raised into crator- 

 like cones to enable the sponge to clear itself of the excremcn- 

 tial matters carried out by the centrifugal streams ; but wlicn 

 it hangs pendant from the rock, the oscula do not rise beyond 



