198 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
ill shallow arms of the sea, lead to the destruction of many. Considering- the phases of 
the life-history of the soft clam thus far described, it seems that artificial methods 
might be developed which should remove some of the dangers to be found in nature, 
and hence lead to a greater proportionate increase in the number of adults. 
BURROWING INTO THE MUD. 
The migration from the point of attachment having been accomplished, we are 
next concerned with the habit of burrowing into the mud. In the adult clam the foot 
is reduced to a laterally compressed, fin-like projection from the anterior side of the 
visceral mass, not extending to its ventral surface. It is with great difficulty that 
the mature clam buries itself in the sand after having been dug from its burrow. 
Clams 1 to 2 inches in length will cover themselves gradually in the course of 30 
to 45 minutes, but they reach the usual depth of several iuches only after a much 
longer period. Very large clams out of their burrows are still less active. In the 
young the foot is relatively very much larger than in the adult, and extends from .the 
anterior side of the visceral mass, just under the mouth, far back on its ventral side. 
This condition of the foot is almost exactly like that to be found in such a clam as the 
quahog ( Venus mercenaria) in its mature state. My a has probably descended from 
an ancestral form which possessed this plowshare-shaped foot, the organ being 
reduced to its present form because it became less and less an organ of locomotion and 
was used simply for digging downward into the sand. We have a confirmation of this 
view in the structural peculiarities of the foot in the very small My a, as described above. 
In the young Mya the foot is capable of great extension, and is used not only in 
crawling over objects, but also in digging into sand and mud. It is extremely inter- 
esting to notice that individuals but 1.5 mm. loug, when placed upon sand, at once 
attempt to cover themselves by thrusting and worming the sharp anterior end of the 
foot into it. Unless the sand be extremely fine, clams of this size are not able to 
thrust aside the grains sufficiently to obtain a lodgment. Those measuring 2 or 3 mm. 
in length are sometimes able to cover themselves partially or wholly; while one 0 mm. 
long can usually work its way beneath the surface of any clam bed, and thus rest in 
comparative security. All clams which I have observed, under 6 or 8 mm. in length, 
work their way downward only far enough to cover the shell. None of them seem to 
be very energetic, and often after working loug enough to raise the posterior end of the 
shell to a vertical position, they give up the attempt to bury themselves and remain 
in that attitude until toppled over by the water currents. After having become 
completely covered, they exhibit a great deal of restlessness, apparently often push to 
the surface again, as if dissatisfied with the surroundings, and after wandering about 
for a short distance once more go down. This process has been seen to be repeated, in 
an individual 0 mm. long, half a dozen times in the course of three days. They seem 
to wander short distances — 1 or 2 inches only — between the periods of descent, but 
perhaps the wanderings on the bottom may at times be more extensive. 
How large the clam becomes before it digs into the bottom to remain permanently 
has not been determined. I have frequently found lying on the surface empty shells 
at least 2 cm. loug which had been perforated by the oyster-drill ( Urosalpinx ), and 
this creature could only have made its attack when the living clam was out of its 
burrow. Clams of this length, then, apparently have periods of wandering, and it 
would be interesting to determine, if it were possible, whether or not they would be 
able to move up between the tide marks from some position below low tide. When 
