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 by 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 Leucodore to be a chemical 
one, has any acid been observed? It has: specimens of 
Leucodore, Paced 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 Lewcodore is a little clear sac containing clear vesi- 
cles: its function and homology are doubtful (fig. 8); it may 
ossibly secrete an acid fluid. But it seems much more pro- 
ble 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- 
huric 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 mollusk can 
exude affords a complete protection to any surface from further 
_ erosion by the acid. One argument in favour 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 Serpula we have a dense calcareous shell depo- 
sited by the surface 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 court in 
all cases of ge eee and whilst, in the cases of Sabella and 
Leucodore, I believe the greatest effect must be attributed to 
