. 
14 The American Naturalist. [January, 
a result, both muscles are of about the same size. “The condition 
described is that existent in the completed prodissoconch stage 
in all Pelecypods, as far as known. In Tater life, however, a 
revolution of the axes of the soft parts may take place, so that the 
antero-posterior axis (represented by a line drawn through the — 
mouth and middle of the posterior adductor muscle), instead of © 
being parallel to the hinge axis (the axis of motion of the valves) _ 
as in dimyarians, may present a greater or less degree of divergence 
from the parallel. In progressive series, as in Modiola (Fig. 3), i 
Perna, etc., as the anterior adductor is brought nearer and nearer 
to the hinge line, where its mechanical action is less and less — 
effectual in closing the valves, we find that it is more and more — 
reduced until it finally disappears from disuse and atrophy, as in 
Ostrea (Fig. 4), and Pecten. Conversely, the posterior adductor in : 
the same series in the revolution of the axes is pushed farther and x 
farther from the hinge line and nearer to the central plane of the 1 
valves, where. its mechanical action is most effectual in closing the 3 
valves. With its increase in functional activity the muscle = 
increases in size. The revolved position of the axes, and the con- a 
sequent reduction or loss of the anterior adductor and increase of — 
the posterior adductor, is found in many widely separated genera 4 
of Pelecypods, as Ostrea, Mulleria and Tridacna; thus proving 
the development of the same features on different lines of descent? 
In Aspergillum the two valves have concresced so as to forma ‘ 
truly univalvular, tubular shell, so that the adductors would — 
evidently be functionless if existent. The posterior adductor has — 
disappeared and the anterior is reduced to a few disconnected 
shreds (Fischer), though evidently existent in the young, as 
attested by the form of the shell in the nepionic stage. 
The condition of the foot in Pelecypods depends largely upon 
its use or disuse. In free-crawling or burrowing forms the foot 
is highly developed, and is sometimes of a peculiar form which — 
could doubtless be traced to special functional uses. In Pecten, — 
in the nepionic period, the foot is highly developed, and actively 
ê Dr. B. Sharp and I published almost simultaneously closely similar views on the 
a the relative size of the adductors. See Proc. Acad. Nat. Sci. Phila., : 
1988, p. 122, and Proc, Bost. Soc. Nat. Hist., Vol. XXIII., 1888, p. 538. 
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