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latter case being of the most corrosive and energetic description. If any 

 such chemical was contained in the body of a s;")onge, I do not think we 

 should expect to find siliceous spicules in it. It has been urged that probably 

 the carbonic acid evolved in the respiration of the sponge would be sufficient 

 to dissolve the lime of the rock or the shell, but to this I would ask, suppos- 

 ing this to be the case, that would not account for the removal of the silica 

 in a flint-boring ; and has it been proved that sponges exhale carbonic acid 

 as other animals do, and would carbonic acid be evolved only at the point of 

 excavation, which is not at the osculum, but at the tail, if we may suppose 

 it to have one. The theory I would advance, and which would be one em- 

 bracing modifications of b>th these preceding theories is, that boring.sponges 

 have a limiting membrane endowed with the power of absorption, and I 

 have been led to this view by an examination of the characters of the Cliona 

 borings, and a comparison of them with the well-known absorptive action 

 which occurs in the human subject, and which leads to the shedding of the 

 milk teeth in children ; and, to illustrate this theory, I have placed near my 

 microscope for comparison, specimens of the absorption of the dentine of 

 the temporary teeth, and side by side with that the inner surface of a Pecten- 

 shell, which was the seat of a Clional growth, and 1 hope the examination 

 of the specimens will prove by their character that the tube of a Cliona is 

 produced by a similar absorption to that which gradually causes the shed- 

 ding of a child's first tooth, where we know that a soft cellular papilla grows 

 up in close contact with the root of the temporary tooth, and absorbs the 

 dentine into circular depressions similar in character with those shown as 

 the interior of the borings of Cliona; and the analogy is further strengthened 

 by the fact that it is by no means an uncommon thing to meet with cases in 

 the temporary teeth where an arm of these papilla, having a more energetic 

 action than the rest of the surface, has pushed ahead and bored its way into 

 a chamber for itself. My conviction that Cliona borings are not due to 

 mechanical action, has been much confirmed by the perusal of a paper by 

 Prof. Martin Duncan, published in the " Transactions of the Royal Society," 

 for 1876, in which he shows that a Thallophytic growth had permeated the 

 septa and outer walls of certain Madrepurar'ia, and I had an opportunity 

 afforded me for inspecting the specimens from which he had drawn his obser- 

 vations, and the soft mycelium threads of this fungus, unprovided though they 

 were with any mechanical means of boring, had yet perforated the hard struc- 

 tures of these corals, and were even fructifying within the pent-up walls of 

 their prison-house. Now here were no diamond-edged spicula to bore their 

 way with, and no Annelid of the diameter of l-10,00()th or l-20,000th of an 

 inch could have prepared the way for them which they so accurately filled 

 up, but it may be probable that the carbonic acid, evolved by the extremity 

 of the growing plant, may have acted on the hard arragonite of the coral. 

 It is not for me to measure swords with such doughty warriors as Mr. 

 Waller and Mr. Stewart, but I confess my inability to accept the mechanical 

 and chemical theories advocated by these gentlemen, and simply suggest a 

 method by which this boring, or rather excavation, may be accomplished. 



