326 THE OCEAN WORLD. 
it suffices to explain the excavation of the galleries, however extensive 
their ramifications. Again, the upper cutaneous folds, especially the 
cephalic hood, having the power of expanding at will by an afflux of 
blood, being covered with a thick coriaceous epidermis, and moved 
by four strong muscles, seems to me very capable of performing the 
operation. It appears very probable that it is this hood which is 
charged with the remova! of the woody fibre, rendering it incapable 
of resistance by previous maceration, which may also be assisted by 
some secretion from the animal.” That the fleshy parts of the mollusc, 
acting upon the surface, softened 
by long maceration in water, is 
the true boring implement em- 
ployed by the Teredo, is, pro- 
bably, the only explanation the 
case admits of; at all events, in 
the present state of our know- 
ledge, the explanation of this 
naturalist is the most reasonable 
which can be given. 
The engraving (Fig. 129) re- 
presents Pholas dactylus, which 
has hollowed itself a home out of 
a block of gneiss. This dwelling 
is a cell just deep enough to con- 
tain the animal and its shell 
(Fig. 130). To excavate its cell 
at the bottom of one of these 
gloomy retreats seems to be all 
B that the animal lives for. To 
Fig. 130.—Pholas dactylus (Linnzus). ascend to the summit or sink to 
the bottom of their narrow house 
makes up all the accidents of existence to these strange creatures: 
the hole they dig is at once their dwelling and their grave; which 
fact is attested both by the rocks of the past and the present. 
In its structure the shell of this genus differs notably from other 
Acephalous Molluscs, which led Linneeus to place it in a section 
which he made of multivalve shells. Between the two ordinary 
valves, in short, this shell presents certain accessory pieces, smaller 
than the true valves, and placed near the hinge, as represented in 
Pholas dactylus (Fig. 129), pieces which would not be there without 
a purpose. 
The shell is equivalve, gaping on each side, swelling below, very 
SS 
SS 
