SINGLED-RAYED SPONGES 



3519 



which is thus enabled to dissolve the carbonate of lime of the shell or limestone; 

 but an objection lies in the fact that carbonic acid is 1 incapable of dissolving the 

 organic plates of shells. Recently it has been urged that the power of contractility 

 possessed by the sponge is a powerful aid in the work of excavation. Burrowing 

 sponges are a trouble in oyster culture, and it is suggested that at the time when 

 the free-swimming sponge embryos are formed, a bank of old shells should be placed 

 between the oyster beds and the tide. The bank would filter off the embryos, which 

 would grow in the old shells, and be subsequently destroyed by immersion in fresh 

 water. A figure of a fragment of limestone thus perforated by sponges is given on 

 this page. 



Fresh- Water To the group under consideration belong the fresh-water sponges 



Sponges (Spongillidce'} , which live in ponds, canals, lakes, and rivers all over 



the world; and have been known to infest the pipes supplying a city with water. 



UMESTONE BORED BY SPONGE. 



The two commoner British species (Euspongilla lacustris and Ephydatia flumatilis) 

 grow on the piles of bridges, the sides of locks, the stems of waterweeds, or 

 form crusts on the bed of rivers. Euspongilla forms bright green crusts, from 

 the surface of which long, simple, or branched stems arise; or the surface of the 

 crust may be simply conulated. This green color is due to granular bodies which 

 crowd the cells near the surface of the sponge. Some naturalists consider these 

 bodies to be chlorophyll granules similar to those of plants; others regard them as 

 single-celled algae. The chlorophyll, in the presence of sunlight and water, splits 

 up the carbonic acid evolved by the sponge into carbon and oxygen, the latter 

 being used by the sponge for respiration. Fresh-water sponges growing in shady 

 places are of a pale gray or yellowish- white color; and when bright green 

 specimens are kept in the dark, they lose their green color. The surface of 

 a fresh-water sponge is covered with fine pores, while here and there a few 



