768 Transactions of the Society. 



tions without detecting the presence of an acid, and Professor Stewart 

 has given me the opportunity of examining, under the Microscope, 

 sections of the cavities produced by the boring sponge. The posi- 

 tion occupied in these sections by the siliceous spicules makes it 

 certain that they cannot be the agents of erosion, at least in a 

 mechanical way. In the sections we can see everywhere small 

 hemispherical excavations of the shell occupied by extensions of the 

 protoplasm of the sponge. Yet, at many points, long attenuated 

 conical cavities are seen invading the substance of the shell ; always 

 filled with the protoplasm, often containing one or two siliceous 

 spicules, which from their length and position must have been 

 totally incapable of exerting any mechanical action. As it seems 

 to me, we have here a transposition of the conditions which in the 

 experiments related at the beginning of the paper brought about 

 the breaking-up of the glass surface. There we had a colloid con- 

 taining carbonate of lime applied to a glass surface ; here we have a 

 colloid containing silica applied to a surface of carbonate of lime 

 mixed with another colloid. I venture to believe that in the case of 

 the boring sponges, the action upon the shell is of compound nature ; 

 consisting partly in the contact of a new colloid, partly in the 

 addition of the presence of a different crystalloid. Examining 

 carefully the sections which Professor Stewart has lent me, I am able 

 to recognize in the substance of the protoplasm adjoining the excava- 

 tions, delicate spherules having all the appearance of carbonate of 

 lime re-arranged in a new matrix. 



I have cited here but a few illustrations of the possible application 

 of Mr. Piainey's observations to explain the formation, and stUl 

 more, the removal or absorption of shells and shell-like substances. 



I venture to hope that the attention of this Society will be more 

 and more drawn to the contemplation of these subtle non-chemical 

 agencies as factors in the process of building and repair in the hard, 

 and, very probably, in the soft -structures of animal bodies. 



