360 
Mr. Osler on burrowing 
at the upper margin of the posterior adductor ; and passing 
forward and downward, meet near or upon the body of the 
animal, and advance together along its posterior edge. The 
foot itself is embraced by fibres given off from all these 
muscles. 
A tube opens just within the mouth, going off nearly at a 
right angle with the oesophagus : it proceeds in a curvilinear 
direction through the body, immediately above the foot : its 
course is indicated in the figure by the dotted line at c. It is 
this tube which conveys the water by which the foot is dis- 
tended. In some of the largest species, as Cyprina islandica, 
a transverse section of the foot shows a single chain of pores 
along its whole length, which communicate with the prin- 
cipal tube, and transmit the water to the cellular portion of 
the foot. 
It is scarcely necessary to detail the action of these several 
organs. The point of the foot being solid, is adapted to pre- 
cede the distended portion in penetrating the sand. A viscid 
matter is secreted from the surface of the foot when dis- 
tended, which, by agglutinating the sand around it, fixes 
it more firmly, and thus augments the force of the stroke. 
The operation of the anterior and posterior muscles of the 
foot cannot require explanation. 
The locomotive Bivalves travel by a series of motions pre- 
cisely similar to those employed in burrowing, and which are 
effected by a similar arrangement of muscles. The foot, 
instead of being pointed downward, is projected horizontally 
just beneath the surface of the sand ; and the shell is brought 
forward by the successive contraction of the anterior and the 
posterior muscles. The Venus gallina is particularly dis- 
