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and boring marine animals. 
The Pholas is provided with four sets of muscles, in addi- 
tion to those belonging to the syphon. The posterior adductor, 
(fig. 4, c,) is a flat muscle, connecting the valves nearly 
midway between the hinge and the extremity of the shell, 
and having its attachments at about of an inch within its 
dorsal margins. It lies so superficially, that its action is seen 
when the animal employs it in boring. The anterior adductor 
( a ) is attached to the reflected folds which cover the umbo. 
It extends from a point a little before the hinge, nearly to the 
anterior extremity of the shell, and is covered by the acces- 
sory valve. The ventral margins are connected by mus- 
cular fibres, from the opening in the mantle through which 
the foot is projected, as far as the origin of the syphon. A 
pair of muscles, which may be termed “ lateral/’ arise from 
the points of the long, hooked processes, which, becoming 
fan-shaped, as they pass over the body, are inserted into the 
sides of the foot. 
The offices of these muscles are peculiar. The shell is 
closed, not by the adductors, but by the fibres which connect 
the ventral margins of the valves ; and it is opened by that 
part of the anterior adductor which lies nearest to the hinge, 
and which thus performs an office analogous to that of the 
ligament in other Bivalves. The other portion of this muscle 
antagonizes the posterior adductor. By its contraction, the 
anterior points of the valves are brought into contact, and 
their dorsal margins separated as widely as possible. The 
action of the posterior adductor reverses this state ; and, in 
uniting the dorsal margins, expands the anterior and armed 
portion of the shell. The foot of the Pholas, like that of the 
Gasteropodes, is a flat disc, by which the animal can attach 
