1891.] Mechanical Origin of Structure in Pelecypods. 13 
plest condition mechanically possible to effect the desired end.’ 
This muscle does not seem to be homologous with any muscle in 
other classes of molluscs, and is probably developed from the 
mantle muscles as a consequence of the conditions of the case. 
In support of this view bivalvular crustaceans may again be cited. 
They have an analogous adductor muscle, developed of course on 
an entirely different line of descent, but under closely similar 
mechanical conditions. At the completed prodissoconch stage 
in all Pelecypods, as far as known, there are two adductor mus- 
cles,a second one having developed in the posterior portion of 
the body. In later life the anterior, the posterior, or both ad- 
ductors may be retained, reduced, or lost, according as the per- 
sistence or changes in correlated anatomical features retain in use 
or bring into disuse the muscles in question. 
e 
ao —Mya arenaria. FIG a E eare Peata: FIG. 4.—Ostrea eiepinient: 
ip ax, antero-posterior eo hinge axis; aad, anterior, and # ad, 
posterior WLA muscle ; m, mouth ; %2, Bni a, anus ; g, gills; gd, pedal musc J. 
yssus ; #, 
Let us look at examples of the retention or loss of the adduc- 
tors. In typical dimyarian Pelecypods, as Mya (Fig. 2) or Venus, 
the adductors lie toward either end of the longer axis of the shell. 
As the hinge occupies a position on the borders of the shell about 
midway between the adductors, both muscles are nearly or quite 
in a position to be bio functional in closing the valves. As 
vane in areata See pp. pre Trans. pA Free Institute, Vol. III., Aug., 1890. 
3 Characteristic of young Dentalium., 
* Characteristic of the extreme young of cephalous molluscs, 
5 This early adductor appears in the same position in many genera, and is apparently 
charact of the class. vialen e TE O a in later stages; 
but it may be retained or lost in the adul 
