LOCOMOTION IN GASTROPODS. 
847 
In our Lsomyate species tlie pedal retractors, which serve for the 
withdrawal of the foot, consist of two pairs of powerful muscles, 
aftixed to the right and left valves at opposite ends of the shell, near 
to or continuous with the adductors, and distinguished as the Anterior 
or Posterior pedal retractors, according to the position they occupy. 
In some exotic Monomyate species, as Pecten, the right and left pedal 
Fig. 640. — Left valve Aiiodonta cygnea (L.), showing the position and character of the pedal 
musculature and the arrangement of the adductors characteristic of the Isomya. 
a.a. anterior adductor ; p.a. posterior adductor ; a.r. anterior retractor of the foot ; /.r. posterior 
retractor ; ti,r. umbonal retractors or pedal levators; a.p. anterior protractors ; /. foot. 
retractors are both affixed to the left valve and not eiiually distributed 
to both as is usual, or as in the byssiferous species the posterior 
retractors may, owing to the degeneracy of the foot, become trans- 
formed into the byssal retractors. 
In Dreissensia, our only representative of Heteromya, the anterior 
pedal retractors are greatly attenuated and distally fixed to the 
umbonal region of the shell, 
the constituent fibres being 
distributed to the anterior 
base of the foot. The 
posterior retractors are 
correspondingly enlarged, 
attaining a comparatively 
enormous size, their posi- 
tion being indicated by an indistinct oblong scar of attachment near 
the superior margin of the valves and anterior to the scar of the 
posterior adductor; they have, however, become partially transformed 
into byssal retractors. 
The Locomotion or progi-ession of terrestrial testaceous Gastropods 
is more tedious and laborious than that of the a(iuatic forms, owing 
Fig. 641. — Left valve of Drc’issfns/a polymorpha 
(Pallas), showing the muscular system characteristic of 
a byssiferous Heteromyan. 
a.a. anterior adductor : p.a. posterior adductor ; a.7‘. 
anterior pedal retractor; p.r. posterior pedal retractor, 
also functioning as retractor of the byssus. 
