THE ANATOMY OF TIIE HIYEIt-MUSSEL. 
245 
placed by Lamarck in a division by themselves, under the name 
of Monomyaria , to distinguish them from the Dimyaria , or 
bivalves with two adductors. This single adductor answers to, 
or is “ homologous ” with, the posterior of the two adductors 
of Anodonta .* 
2. The foot, which may be regarded as a muscular mass, 
hollowed out for the lodgment of the digestive and reproductive 
organs, possesses a distinct set of muscles for its protrusion and 
withdrawal. The protrusion is effected by a fan-shaped muscle 
— the “protractor pedis” — whose fibres radiate to either side 
of the foot mass from a point in the valve situate to the ven- 
tral side of the impression of the anterior adductor (fig. 1, 
p p). There are two pairs of retractors. The anterior pair 
converge on either side from the edge and anterior portion of 
the foot to the dorsal portion of the side of the anterior ad- 
ductor. The posterior retractors are two muscular stems which 
pass down on either side from the posterior portion of the foot, 
nearly parallel with the dorsal margin of the shell, to be 
inserted in the valve at the anterior and more dorsal portion 
of the impression of the posterior adductor (fig. 1 , r p p). 
A few retractor fibres also arise a little in advance of the umbo 
on either side, and pass to the dorsal region of the foot (fig. 
i, r p a). 
If a small piece of one of the adductors be teazed out with 
needles on a glass slide, and be then placed under a ^-inch 
power, it will be seen to be made up of unstriped fibrils, in 
which nuclei appear to be brought into view, after treatment 
with acetic acid. 
In the fresh-water Dreissena , as well as in the marine mussel 
(. Mytilus edulis\ the foot is very rudimentary, being mainly 
subservient to the secretion of that peculiar bundle of threads, 
the “byssus,” by which the animal anchors itself to various 
bodies.f It is a curious fact that the Dreissena has been seen 
to break the threads by which it was attached, and creep about 
by stretching and shortening its foot, somewhat in the manner 
of a leech. The left valve of the specimen from which fig. 1 
was drawn had a byssus of this mussel attached to its exterior, 
which may have become detached from the animal, in the 
manner above described, during life. 
* It is a curious fact that a species of fresh-water mussel (Miilleria) found 
in the neighbourhood of Bogota, S. America, is locomotive and di-myary 
when young, though fixed and mono-myary when adult. 
t That greatest of naturalists, as well as of moral philosophers, Aristotle, 
was well acquainted with this structure as existing in the Pinna of the 
Mediterranean. He says ( De Anim. Hist. lib. v.), At di Trivvat op9ai tpvovrcu 
£K Toil filMTVOV tv roig apu'jjdtoi Kcii (3oofiopuidtoii\ 
