BRITISH SPONGES 125 



matter are divided in opinion as to whether the work is done by 

 chemical or by mechanical action. 



Some of those who advocate the chemical theory suppose that an 

 acid fluid is secreted by the sponge, and that the carbonate of lime 

 forming the shell or stone is thereby dissolved ; but such advocates 

 have, as yet, failed to detect the presence of any acid substance in the 

 body of the animal. Others ascribe the action to the solvent power 

 of carbonic acid gas. This gas certainly has the power of dissolving 

 carbonate of lime, as may be proved by a very simple experiment : 

 Pour a little lime water into a glass, and blow into it through a glass 

 tube. The lime water speedily becomes milky in appearance, the 

 lime having been converted into particles of chalk or carbonate of 

 lime by union with the carbonic acid gas from the lungs. Continue 

 to blow into the liquid for some time, and the carbonate of lime 

 will slowly disappear, being gradually dissolved by the excess of 

 the gas the gas over and above that required for the formation of 

 the carbonate. Thus, it has been said, the carbonic acid gas 

 evolved as a product of the respiration of the sponge is the agent 

 by which the channels are excavated. Whatever be the acid to 

 which this power is ascribed, whether it be the carbonic acid or a 

 special acid fluid secreted for the purpose, there is still this 

 difficulty in the way of accepting the theory, namely, that an acid, 

 though it has the power of dissolving the mineral matter of a shell 

 the carbonate of lime has no action on the laminae of animal 

 substance that form part of the structure. If we put the shell of a 

 mollusc in hydrochloric or dilute nitric acid, we obtain, after the 

 complete solution of the carbonate of lime, a substantial residue of 

 animal matter which the acid does not touch, but in the case of 

 Cliona both animal and mineral sub- 

 stances yield to its power. 



Those who favour the mechanical 

 theory assert that the material is worn 

 away by siliceous particles developed by 

 the sponge, and kept in constant motion 

 as long as the animal lives; and the 

 theory is supported by the statement that, 



in addition to the spicules of silica, which 



, , , ,, , . FIG. 76. SPICULES OF 



are pin-shaped, and occupy the interior Cliona 



of the animal, there are little siliceous 



granules scattered on the surface of the sponge which are kept in 



constant motion resembling that of cilia ; and the minute particles 



