Animals Before Man 



at work constructing barrier reefs and coral 

 islets. 



Then come the Porifera, or sponges, for so 

 long a time bandied about between the animal 

 and vegetable kingdoms, but now definitely 

 located in the former. This division contains 

 several orders, which, with one exception, have 

 an interior framework of glassy or horny fibers, 

 and it is this framework that forms the sponge 

 of commerce, all the living matter having been 

 removed. Sometimes the former existence of 

 sponges is revealed merely by the presence of 

 spicules, or portions of the skeleton ; sometimes 

 these have held together and retained the gen- 

 eral outline of the entire sponge. A living 

 sponge is really an assemblage of numerous 

 individuals disposed about a common cavity, 

 provided with one or more openings through 

 which the water passes, and, small as are the 

 individuals, yet their combined action causes 

 such a current that on a calm day the presence 

 of a large sponge may be revealed by the mo- 

 tion of the water. Huxley has compared a 

 sponge to a "kind of subaqueous city, where 



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