PERFORATIONS OF MARINE ANIMALS. 55 
—by devouring all the wood it eroded—could not long satisfy 
more philosophic inquirers, and accordingly many able memoirs 
on the boring of these and other marine animals have enriched 
zoological literature during the last century. In briefly alluding 
to these the perforations in various hard substances will be in- 
cluded, in order to illustrate the several features of the action. 
It will be found that the theories which have been brought 
forward to explain the mode by which marine animals perforate 
materials so different as wood, limestone, resin, wax, granite, 
sandstone, and aluminous shale, range themselves round two 
great centres—the chemical and the mechanical, for it is un- 
necessary to dwell on such as those of a few, who fancied that 
Pholas, for instance, only bored in soft clay, and that its presence 
in stone was due to the petrifaction of the soft materials around 
it. This explanation is comparable to that of the religious 
authority mentioned by Hugh Miller, who accounted for the 
beautiful fossil fishes in the rocks by courageously asserting that 
‘they were formed of stone from the very first.”’ 
The advocates for the chemical theory seem to take it for 
eranted that the borings occur chiefly in calcareous substances, 
and with propriety, therefore, they make their solventan acid. It 
is clear, however, that this notion is unable to explain the per- 
forations in media totally impervious to such action. Moreover, 
no trace of acid is found in many borers, and though present in 
some it is likewise characteristic of other marine animals which 
do not bore. Further, it is purely hypothetical at present to 
bring in the aid of carbon-dioxide derived from sea-water for 
the same reason. 
The mechanical theory, again, supposes that the animals 
perforate by means of shells, gritty particles, or odontophores in 
the case of mollusks, teeth in the sea-urchins, bristles in the 
annelids, horny processes in certain sea-acorns (Cirripedes) and 
Gephyreans ; but doubt remains concerning the extensive and 
wonderful excavations of the sponges, those of the Bryozoa, the 
Helices, and the rest of the Cirripedes. If, however, the theory 
of ‘“‘ maceration” is regarded as a modification of the foregoing, 
certain objections will disappear. The grains of wood found in 
the stomach of Xylophaga and Teredo are, however, interesting 
in this connection. 
