346 
KETRAPTORS IN PELECYPODA. 
tion of the posterior adductor ; while iii the IMonomya (/xoros, one ; 
/ii's, niuscde) of which tlie (t3'ster is an example, the anterior adductor 
has become completely atrophie<l and lost, the enlarged posterior 
adductor assuming a more central position in the shell. 
The adductors are distinguished as Anterior and Posterior, terms 
expressive of their relative positions in reference to that of the 
animal inhabitant. 
The Anterior adductor is most dor, sal in position and placed in 
front of and in constant association with the mouth, and is the first 
developed in the embryo, the more 
ventrally placed Posterior adductor 
being ventral to the rectum with which 
it is always in close relation ajipear- 
iiig later, but their ])oints of fixation 
to the shell gradually become more 
and more ventral in correlation with 
the growth of the shell, their hulk 
also correspondingly increasing, the 
Kig. 630. — Em})ryo of Anodonta seen 
from the left side, enlarijed (after Gutte), 
showing the early origin of the anterior 
adductor. 
a. a. anterior adductor ; hi. blastopore ; 
c. ciliated area ; sh, shell. 
newly-formed portion being generally distingui.shable by its lighter 
colour. 
The adductors act by closing the valves of the shell, a poidion of 
their substance lieing composed of involuntary or smooth muscle 
fibres, and the residue may he the pseudo-striate fibres characteristic 
of molluscan voluntary muscle ; the valves, owing to the antagonistic 
action of the ligament, tend to remain slightly oj)en when the 
muscles relax, this position being their state of rest or equilibrium. 
The strength of the adductors is very great and, in certain cases, 
tliej' have been known to resist a force many thou, sand times greater 
than the weight of the animal it,self. 
The liETRACToRS of the Pelecypoda are symmetrically arranged 
and paired muscles, strongly attached to corresponding areas of the 
concave inner surface of each valve of the shell, on which they leave 
l)ermanent traces of their i)re,sence in the form of scars, the constituent 
fibres being distribiued within the foot or other organs to which they 
may belong. In the archaic genus Nucula, the retractors emanate 
as usual from each valve and form an almost continuous series of 
muscular bundles between the adductors, but in more .specialized 
forms thej" become concentrated, each muscle occupying a definitely 
circumscribed area of the shell surface. 
