HOW SPONGES LIVE. 39 
Having settled, the young sponge now spreads 
itself out upon the rock, and grows and builds up its 
fibrous skeleton, while its surface becomes irregular 
and full of large and small holes, and the true sponge 
appears. 
And now comes the curious part of the story. 
As the sponge grows larger it is clear that the cells 
in the middle of its body must be more and more 
Fig. 12. 
Development of a young English sponge.* Adapted from Carter. 
3, The swimming sponge of Fig. 1 1, which has now fixed itself. 4, 
The same with water squirting from the hole now formed. 5, The 
same further developed. 6, The perfect sponge with small holes, 
where the water enters, and large holes out of which it is squirted. 
shut out from the surrounding water out of which 
food can be taken, and yet these cells want feeding 
as much as those outside. In order to bring this 
about, the sponge-animal, instead of growing up as a 
solid mass of slime-cells, arranges the silky fibres of 
its skeleton in such a manner as to leave a number of 
small canals or passages throughout its body, and these 
open, as we have seen, sooner or later, into large canals 
* Halichondria simnlans. 
