194 INVERTEBRATE ANIMALS, 
quite free ; but at other times they are more or less united to 
( 
: 
M 
f 
NN), 
i UN Re 
VW Ss 
Fig. 100.—Anatomy of a Bivalve Mollusc. 
Mya arenaria (after Woodward). The 
left valve and mantle-lobe, and half the 
siphons, are removed. ss Respiratory 
siphons, the arrows indicating the direc- 
tion of the currents; a @ Adductor muscles; 
6Gills ; 2 Heart ; o Mouth, surrounded by 
(p) labial palpi;_f Foot ; v Anus; az Cut 
edge of the mantle. 
each other, and leave only two 
openings. Through one of 
these openings (the anterior) 
the “foot” is protruded (fig. 
Ioo, 7); and through the 
other pass the respiratory 
tubes or “siphons” (s). The 
foot in the bivalves is a 
muscular organ developed 
upon the lower surface of 
the body, but not forming a 
creeping flattened disc, as in 
the ordinary univalves. In 
many Cases it is quite rudi- 
mentary ; and even when it 
is employed in locomotion it 
is usually small. Most gene- 
rally it is hatchet-shaped or 
pointed (fig. 100, /),and serves 
to enable the animal to make 
short leaps. In many cases 
—as in the common mussels 
—the foot is subsidiary to a 
special gland, which secretes 
a viscous fluid, which hard- 
ens rapidly on exposure to 
the air. This fluid is mould- 
ed by the foot into silky 
threads (the so-called “ bys- 
sus”), by means of which the 
shell is firmly fixed to some 
solid object. Besides the 
muscular foot, other muscles 
are present as well in the 
Lamellibranchiata. Of these 
the most important are the 
muscles which close the shell, 
and are called the “adduc- 
tor” muscles. - In one group 
of the bivalves (fig. 101, 3) 
