THE BRITISH SPONGES. 13 



we are told that all excremental matters are carried out, — 

 every thing to wit which has become effete, or which has been 



Fig. 6. 



rubbed down by exposure to its force. The direction of the 

 cmTents is said to be invariably the same ; and a sortof ara- 

 neous web is described as being spread within the oscula, 

 calculated to prevent the ingress through them of any fo- 

 reign bodies floating about, wliich might otherwise disturb 

 the functions of the sponge by blocking up these conduits 

 and vents. There are, however, many sponges which are 

 entirely destitute of oscula, and whose surface is equally 

 and miiformly porous ; and the pores of these, we are com- 

 pelled to believe, may be, at different periods, and under va- 

 rying circumstances, either suctorial or vomitive. Dr Grant 

 describes the influx and efflux of the water as constituting 

 an uninterrupted stream, flowing as long as the sponge con- 

 tinues to be submerged and in health ; and, according to 

 the same authority, it is ofttimes strong enough to be dis- 

 tinguishable with the naked eye, — especially if any light 

 powder is strewn over the siu-face of the water to guide the 

 eye to its coiu-se and origin. 



This physiology of the sponge we owe to Professor 

 Grant. The theory is so consistent with the structure and 

 low organization of the object, and is deduced from such 

 an extensive series of w^ell-told experiments, that it made 



