148 VIGNETTES FROM INVISIBLE LIFE. 



the river or life-current of its continuous circula- 

 tory and respiratory functions, on which its very 

 life depends from moment to moment. 



Contemplating this miniature Venice-like city, 

 one feels that something more is requisite to 

 render it complete ; for, although unlike the City 

 of Waters to which we have compared it, in 

 this, that its living activities are all inclosed, 

 opening in upon themselves, and exposing only 

 their basal structure to the external world, yet, 

 even so, a defence and protection are essential to its 

 well-being and preservation. And this the Sponge 

 has not neglected ; indeed, it has made defence its 

 especial care, so that a stranger say a Stickleback 

 on approaching it from a distance, sees only these 

 defences, and, if not previously acquainted with its 

 structure, would hardly imagine it to be other than 

 a mass of prickly non-living matter. Its spicular 

 investment is composed of one of the hardest and 

 most enduring substances known, spreading over 

 and imbedded in a soft, structureless, membranous 

 tissue, which it at once upholds and protects. 

 Thus, while tenderly surrounding the living city, 

 and offering the most formidable defence to vio- 



