274 UTILIZATION OF MINUTE LIFE. 



larger varieties which cling to the rocks, well repay 

 observation, and would form interesting objects for 

 the aquarium. The same might be said of those 

 two remarkable species of fresh-water sponges, 

 Spongilla fluviatilis and S. lacustris. One of these 

 species (8. fluviatilis) is not unfrequently met with 

 in the ditches around Paris, and probably around 

 London also. These Spongilla are green, and at 

 first sight would be taken for vegetables. Mr. John 

 Hogg has published, in the "Linnsean Trans- 

 actions," some experiments made with a view of 

 ascertaining the effect of light upon these fresh- 

 water sponges. He has shown that they are influ- 

 enced by it as vegetables are, and that their green 

 colour depends upon their exposure to it. M. 

 Dutrochet, in the memoir cited above, has studied 

 minutely the organization of these fresh-water 

 sponges. 



To return to marine sponges, one of the most 

 common of our indigenous species, Spongia oculata, 

 or Halichondria oculata (Fig. 37), may be made to 

 serve the same purposes as foreign sponges, save 

 for the toilet; whilst H. palmata, H. cervicornis, 

 H. tubulosa, H. simulans, etc., form beautiful speci- 

 mens for the aquarium. 



Carbonized sponge has been long used in medi- 

 cine ; its effects appear to depend upon the small 

 quantity of iodine contained in it, of which, in 



