MOLLUSC A. 
59 
chemical and the mechanical*. The advocates of the former 
seem to take for granted that the borings occur chiefly in 
calcareous substances ; and with propriety, therefore, they make 
their solvent an acid. It is clear, however, that this agency 
is unable to account for the abundant perforations in media 
totally impervious to such action. Moreover no trace of acid 
is found in many borers; and though present in some, as in 
Sabella saxicava and Pholas , it is likewise characteristic of 
other marine animals which do not bore ; and it is purely 
hypothetical at present to bring in the aid of the carbonic 
dioxide derived from sea-water, for the same reason. 
The mechanical theory, again, supposes that the animals 
perforate by means of shells or gritty particles in the case of 
mollusks, teeth in the sea-urchins, bristles in the annelids, 
horny processes in certain cirripedes and gephyreans; but we 
are left in doubt concerning the extensive and wonderful per¬ 
forations of the sponges, those of the Bryozoa, and the rest of 
the cirripedes. If, however, we regard the u macerating ” 
theory as a modification of this, certain of the difficulties will 
be overcome. The grains of wood, however, found in the 
stomachs of Teredo , are interesting in this respect. 
The whole subject of the boring of marine animals, indeed, 
is much in want of further elucidation ; and it is hard to believe 
that the same modus operandi is employed by all. In con¬ 
clusion, the theories may be arranged under the following 
heads (for all the subkingdoms), after Forbes and Hanley 
and Gwyn Jeffreys :— 
I. That in the shell-fish the perforations are made by rotations 
of the valves, like augers (Bonanni, Adanson, Born, 
J. E. Gray, Dr. Fleming, Osier, Forbes and Hanley, 
Cailliaud, Robertson, &c.) ; in the sea-urchins, by the 
teeth (Cailliaud). 
This theory is not supported by an examination of the 
perforations of the sponges, Bryozoa, those of the annelids, 
gephyreans, and cirripedes, nor by a comparison of the 
* We do not here allude to the boring by jaws or tongue (e. g. in Lim - 
noria and Trochus). 
i 2 
