A BIT OF SPONGE. 57 



us. You perceive half-a-dozen large holes or so, each 

 opening on a little eminence, as it were. These aper- 

 tures, bear in mind, we call oscula. They are the exits 

 of the sponge-dominion. But a close inspection of a 

 sponge shows that it is riddled with finer and smaller 

 apertures. These latter are the pores, and they form 

 the entrances to the sponge-domain. 



On the banks of the canal you may see growing 

 plentifully in summer time a green sponge, which is 

 the common fresh-water species. Now, if you drop 

 a living specimen of this species into a bowl of water, 

 and put some powdered indigo into the water, you 

 may note how the currents are perpetually being 

 swept in by the pores and out by the oscula. In 

 every living sponge this perpetual and unceasing cir- 

 culation of water proceeds. This is the sole evidence 

 the unassisted sight receives of the vitality of the 

 sponge-colony, and the importance of this circulation 

 in aiding life in these depths, to be fairly carried out 

 cannot readily be over-estimated. 



Let us now see how this circulation is maintained. 

 Microscopically regarded, we see here and there, in the 

 sides of the sponge-passages, little chambers or recesses 

 which remind one of the passing-places in a narrow 

 canal. Lining these chambers, we see living sponge- 

 units of a type different from the shapeless specks we 

 noted to occur in the meshes of the sponge substance 

 itself. The units of the recesses each consist of a 

 living particle, whose free extremity is raised into a 

 kind of collar, from which projects a lash-like filament 

 known as a flagellum. 



This lash is in constant movement. It waves to 

 and fro in the water, and the collection of lashes we 

 see in any one chamber acts as a veritable brush, 



