394 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
(the figure haying been sketched from an alcoholic specimen), but its position and the 
marginal papilke may be seen. When the foot is contracted, as in the figure, the 
lateral edges of the disc are brought together ventrally, thus making a crease from 
before backward through the middle of the disc. The disc may be so fully expanded 
that the crease sometimes disappears, but this does not often happen. 
The most usual form of foot in lamellibranchs is that typically shown by Venus 
(Fig. 96, PI. xcrvj. It is flattened from side to side and extends in this case along 
nearly the entire ventral surface of the visceral mass (/). Its anterior end is plough- 
share shaped and is greatly protrusible. Instead of being somewhat flat on its ventral 
surface, it is more or less sharp or heel-like (seen in vertical section at/, Fig. 13). In 
My a the foot is much compressed laterally, and projecting anteriorly to the body, is 
slightly sharper above than below (in vertical section at /, Fig. 23, PI. lxxxii). It does 
not extend along the ventral side of the body, or visceral mass, as in Venus, but occu- 
pies a position more like that of Mytilus and Pecten. 
The foot of Mytilus is an entirely muscular, tongue-like organ, flattened dorso- 
ventrally, concave above and convex on its lower surface (Fig. 42/). In Pecten it is 
relatively much smaller, being a short cylindrical projection from the anterior end of 
the visceral mass. In Ostrea and Anomia, which are fixed forms, the foot has entirely 
disappeared. 
The foot serves a number of different purposes, but is generally used as a burrow- 
ing organ. The end of the foot is protruded as a long, narrow tongue, which digs into 
the sand with a worm-like movement, keeping the shell closed as much as possible. 
When it has penetrated to some depth it expands at the end, the retractor muscles 
come into play, and the whole animal is gradually pulled beneath the surface of the 
sand. The forms with the ventral disc are very active. Toldia, when burying itself, 
makes a sharp point of the anterior part of the folded disc of the foot and very rap- 
idly burrows this into the mud. The disc is now widely expanded, forming a firm 
anchor, the foot-retractors contract and draw the body down to the end of the foot, 
in this way quickly covering it. Solenomya has the same habit, and also often swims 
rapidly through the water by using its powerful foot as a paddle. It is stretched out 
anteriorly, the disc opened, and a rapid backward stroke is made. This is repeated 
with great rapidity. One or two lamellibranchs have the habit of creeping on the 
ventral side of the foot, as in gasteropods, but, as far as I have observed it, in the 
adult of one very small form and in the young of Mytilus and Pecten, it is done in a 
very imperfect way, the animal frequently being unable to maintain its erect position 
when crawling over a smooth surface and falling over on its side. 
The foot of many lamellibranchs, as that of Venus or Anodon, is made up of muscle 
fibers, which are irregularly distributed vertically and horizontally, leaving every- 
where spaces which are in connection with the vascular system. Blood being forced 
into these spaces by the heart, causes the extension of the foot. The general direction 
of the fibers in the foot is indicated in Fig. 13, PI. lxxxi, /. In this section a sharp 
separation occurs between the foot and the genital gland above ( g ). Just below 
this the foot shows many transverse fibers ( mus ). From this region, too, three prin- 
cipal bands of muscle fibers extend down toward the ventral side of the foot. Their 
contraction, probably, aids the foot-retractors in drawing the foot up close to the 
visceral mass. Farther back (Fig. 14), in the region of the posterior end of the stom- 
ach, the sexual gland forces its way for a considerable distance down between the 
