302 THE INHABITANTS OF THE SEA. 
therefore completely inadequate for its defence. For better 
security it bores deep passages in submerged timber, which it 
lines with a calcareous secretion, closing 
the opening with two small lids. Un- 
fortunately, while thus taking care of 
itself, it causes considerable damage to 
the works of man. It is principally 
to guard against the attacks of this 
worm that ships are sheathed with 
copper, and the beams of submarine 
constructions closely studded with nails. 
During the last century, the Teredo 
caused such devastations in the dykes which guard a great part 
of Holland against the encroachments of an overwhelming 
ocean that the Dutch began to tremble 
for their safety ; and thus a miserable 
worm struck terror in the hearts ot 
a nation which had Jaughed to scorn 
the tyranny of Philip II., and bid de- 
fiance to the legions of Louis XIV. 
But while blaming the teredo for its damages, justice bids us 
not pass over in silence the services which it renders to man. If it 
here and there destroys useful constructions, on the other hand, it 
removes the wrecks that would otherwise obstruct the entrance 
of rivers and harbours; and we may ask whether these services 
do not outweigh the harm it causes. The pholades also belong 
to the noxious animals; they perforate the walls and calcareous 
jetties which man opposes to the fury of the sea, or raises for the 
creation of artificial harbours and land- 
ing places, destroy their foundations, and 
gradually cause their destruction. 
The foot of the lamellibranchiates 
presents a great variety of form, and is 
found in various degrees of development, 
Pholas striata. 
gradually passing into a rudimentary 
state, until finally it is completely 
wanting in the oyster family. In most 
of those which live at large it is strong 
.Petunculus, 3 : 
a, Foot. and muscular, serving either as an ex- 
cellent spade for speedy concealment in the sand when an 
