349 
and boring marine animals. 
insinuates it into the sand until it is nearly buried. A circular 
motion is now given to the shell, by which its anterior point 
is quickly brought nearly into contact with the foot, and 
immediately returned to its former situation. It thus moves 
on the foot, as on a fulcrum, with a see-saw motion. The 
foot, which had been partially retracted, is again gradually 
projected as far as possible into the sand, when the circular 
motion of the shell is repeated. When the animal is mode- 
rately active, the strokes follow each other at intervals of 20 
or 30 seconds. The apparent progress is at first but small ; 
the shell, which is raised on its edge at the middle of the 
stroke, falling back on its side at the end of it : but when the 
shell is buried so far as to be supported on its edge, it ad- 
vances more rapidly, sinking visibly at every stroke, till 
nothing but the extremity of the syphon can be perceived 
above the sand. 
The instinct which directs the animal thus to procure a 
shelter, operates at the earliest period of its existence. On 
examining a Mya truncata, dug up on the preceding day, and 
which, when grown, will not attempt to burrow, I found two 
young ones, entangled in the folds of cuticle at the extremity 
of the syphon, scarcely more than aline in length, and appa- 
rently but just excluded. Being placed on sand in a glass of 
sea-water, they buried themselves immediately. 
The motions by which the animals burrow are effected by 
two pair of muscles, which arise from the shell, and are 
inserted into the foot. The anterior muscles, (fig. 2, a) 
arise at the upper margin of the anterior adductor, and uni- 
ting immediately below the mouth, pass down together to 
their insertion. The posterior (fig. 2, 6 ), take their origin 
MDCCCXXVI. Z Z 
