262 
and by a line of fibres extending along the ventral border of the 
genital mass and liver on each side. At a point anterior and lateral 
to the posterior foot muscles, a muscle of some prominence, apparently 
in the same series as the line of muscles just described, is attached 
to the shell. 
These muscles are all more or less closely bound together by 
their own fibres and by interlacing fibres, so that movements occur 
that cannot be explained by direct pulls of one or more muscles, 
without constantly bearing in mind that the attachments are all along 
the sides of the foot, and that many, if not most of the muscle fibres 
pull from une part of the body wall to another, without changing the 
relation of the body to the shell. It should also be borne in mind 
that the action of the muscles upon the fluids of the body, more 
especially upon the fluids of the foot, is probably the principal factor 
in protruding the foot, and plays a very important part in other 
movements. 
The posterior foot-muscles, Fig. 6 pfm, are very powerful. They 
are attached to the shell at the bases of the teeth just anterior to 
the posterior adductor muscle, and they send their fibres along the 
walls of the foot to its anterior and ventral borders. The muscular 
flaps are well supplied by fibres from these muscles. These are the 
principal retractor muscles. 
The anterior foot-muscles, afm, are attached to the shell at the 
basis of the teeth just posterior to the anterior adductor muscle. 
These muscles are arranged in three pairs. The anterior pair pass 
posteriorly and ventrally along the sides of the foot and their fibres 
are distributed to its posterior portion. They would seem to aid in 
protruding the foot and to be especially important in executing the 
movement described, where the foot is protruded from between the 
ventral borders of the valves near their posterior ends and makes a 
sweep anteriorly. The middle pair pass ventrally -between the pair 
just mentioned and their fibres are distributed to the postero-ventral 
portion of the foot. The posterior pair pass ventrally and anteriorly 
between both of the pairs just mentioned, and their fibres are dis- 
tributed to the anterior and ventral portions of the foot. This in- 
cludes the muscular flaps. 
The line of muscles attached to the shell ventral to the genital 
mass and liver, x, together with the muscles just anterior to the 
posterior foot-muscles, y, are somewhat scattered in their distribution, 
but their fibres seem to form an imperfect capsule that invests the 
viscera. 
