Mr. E. R. Lankester on Lithodomous Annelids. 237 



tions in mind, such as Annelids will make in the semi- 

 solid silt filling cracks in shale, or else that he has since 

 seen reason to change his opinion ; for he has not produced 

 any such specimen of shale, although then challenged to do 

 so. I submit that the opinion as to aluminous shale, unsup- 

 ported bj any chemical test or specimen, and confessedly only 

 casually noticed, should not be of any weight in the balance 

 against the facts as to the exclusive erosion of limestone which 

 are above recorded. 



Supposing, then, the agency in Leucodwe to be a chemical 

 one, has any acid been observed ? It has : specimens of 

 Leucodore, placed on litmus-paper, give a strong acid reac- 

 tion, besides which the constant evolution of carbonic acid 

 in the respiratory process, and its efficiency as a solvent of 

 carbonate of lime, are well known. At the base of each para- 

 podium in Leucodore is a little clear sac containing clear vesi- 

 cles : its function and homology are doubtful (fig. 8) ; it may 

 possibly secrete an acid fluid. But it seems much more pro- 

 bable that the erosion of the limestone, as a rule, is due to the 

 evolution of carbonic acid. At the same time, these sacs 

 (which exist also in the arenicolous species) may secrete sul- 

 phuric acid, as MM. Panceri and De Luca have lately observed 

 in the salivary gland of Dolium and other mollusks. All 

 chemists know well the powerful solvent effect of water, 

 charged with carbonic acid, on limestone ;*but some zoologists 

 seem unable to realize it. The objection to the action of car- 

 bonic acid has been made that it would continue to dissolve 

 after the gallery was of sufficient size, and that Serpula and 

 Mollusca would by it dissolve their own shells. There is a 

 very simple answer to this, admitting of experimental proof : 

 it is, that the viscid secretion which the Annelid or moUusk can 

 exude affords a complete protection to any surface from further 

 erosion by the acid. One argument in favom* of chemical 

 action in cases of boring generally, which seems to me to 

 have some force, is that in all cases the same surface which 

 deposits a shell, bone, or other such structure, can also reabsorb 

 it. Now in Serjmla we have a dense calcareous shell depo- 

 sited by the sm'face of the body ; why in other cases should 

 not a similar mass of carbonate of lime be absorbed, or exca- 

 vated, by that surface, as in Leucodore'? In Mollusks we 

 know that the shell may be deposited and reabsorbed ; and in 

 Vertebrates the absorption and deposition of bone goes on at 

 the same surface. The case of Pholas boring gneiss must by 

 no means be held to have put chemical action out of com-t in 

 all cases of perforation ; and whilst, in the cases of Sabella and 

 Leucodore^ I believe the greatest effect must be attributed to 



