The Pholas. 529 
ends, and terminated in front by a point ; it is while 
and chalky in its appearance, and the anterior end is 
roughened by numerous sharp spines and tubercles. 
The animal which inhabits this shell bores deeply into 
the rocks of the sea-shore, forming cylindrical holes, in 
which it lives ; and the water which it requires for its 
food and respiration is conveyed to and from the 
interior of the shell by a pair of tubes which reach to 
the outer orifice of its dwelling-place. It is supposed 
that the Pholas is enabled to bore into the hard rock by 
means of its large and strong foot, but this is still a 
matter of dispute. 
There are many other boring shells, most of which 
are related to the Pholas. Some of them burrow in 
rocks, others in wood, and some indifferently in either 
material. Of the wood-borers, the most remarkable is 
the Ship Worm (Teredo navalis), which penetrates deeply 
into floating or submerged timber, and lines the cavity 
of its burrow with a coating of shell. In this way the 
Teredo has often done much injury to piles and other 
woodwork exposed to the sea, and in 1731 and 1732 it 
excited so much alarm in Holland by attacking the piles 
of the great dikes, that even statesmen condescended to 
study its natural history. We must remember, however, 
that in the grand economy of nature even this destruc- 
tive creature has its use ; by penetrating in every direc- 
tion through any floating mass of timber it promotes the 
breaking up of the latter, and prevents the surface of 
the sea from being encumbered with quantities of 
wreck. 
2 M 
