352 Mr. Osler on burrowing 
within the shell ; and the operculum lies flat above it when 
it is projected and extended. A transverse section of the 
foot near the part where it joins the body, shows four consi- 
derable tubes penetrating the spongy portion, and very near 
each other ; three of which are in a line parallel to, and 
almost in contact with the muscle ; the fourth, a little below 
the middle one of the three. By a series of transverse sec- 
tions of the foot, parallel to the operculum, we are enabled to 
trace these tubes ; and to ascertain that they become rapidly 
smaller as they advance, until they are quite lost; the longest 
of them not admitting of being traced quite to the operculum. 
All these tubes are given off at the extreme anterior point of 
the thorax from a considerable one, (fig. 3, /,) which, being 
situated under the muscular floor of this cavity, takes a direc- 
tion to the right side, and running just within the origins of 
the muscles of the trunk ( k ) passes out of the thorax, nearly 
in contact with, and on the right side of the oesophagus. It 
terminates nearly midway between the heart and the rectum, 
(m) opening into a considerable cavity, which has the liver 
underneath, and the membrane enveloping the spire above it. 
When the animal contracts the distended foot, the water is 
seen to flow out between the mantle and the shell on the 
right side. The tube and cavity are easily inflated by a 
blow-pipe introduced into one of the tubes of the foot. 
The moderately distended foot can scarcely be retracted 
within the margin of the shell ; and when fully injected, it is 
elastic, and of a very large size. The cavity which it opens 
into the sand is therefore fully adequate to receive the shell, 
which is drawn down into it by the contraction of the muscle 
of the spire. From the attachment of this muscle, the spire 
