
" 
i 

24 SALT AND FRESH-WATER CLAMS. 
The mud lies closely packed against the walls of the tubes, 
so that nothing is seen but the expanded lips of the siphonal 
orifices fringed with numerous tentacles. When it happens 
that the surface of the water is only a little above these ori- 
fices, a strong current can be distinctly seen to boil up from 
the anal siphon, and another, with a constant steady flow, to 
set into the branchial one.” 1 
On plate 1, fig. 2, is represented a clam in its natural 
position in the mud, showing the extent to which the tubes, 
or siphons, can be extended; and in Fig. 1 a clam is repre- 
sented with one of the shells —the left shell—removed. As 
we remove the shell, we are forced to separate two muscles 
which hold the shells, or valves, as they are called, together. 
The valves are forced apart by an elastic substance that oc- 
cupies the little tongue-shaped tooth of the shell near the 
hinge, and in order to keep the valve together, the clam has 
to exert a constant force by contracting the muscles. The 
moment the muscles relax, the elastie substance forces the 
valves apart, acting as a piece of India-rubber would act if 
placed within the hinge of a door, and the door closed against 
it. Fig. 4, plate 1, represents a section of the valves of a 
clam, showing the elastic substance, L, and the transverse 
muscle, M. 
Having opened the clam, we find lining the shells within 
a thin membrane called the mantle. Its border which fol- 
lows the edges of the shell, is thickened and united, except 
a small slit through which the so-called foot projects. This 
organ has the power of excavating a hole in the mud. Ac- 
cording to one writer, it assumes a variety of shapes while 
digging: “now a dibble or spade, a trepan or pointed grav- 
ing tool, a hook, a sharp wedge.” 
. The abdomen occupies the centre line of the body, and 
forms the principal edible portion of the clam. It contains 
the ovary and liver, —the liver being recognized by its dark 
color. (For the different parts see plate 1, and explanation 
| of the plate.) The mouth of the clam is directly under the - 



