58 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
three teeth are made to stir up and crush whatever is 
contained in the cardiac chamber. When the stomach is 
removed and the front part of the cardiac chamber is cut 
away, the front cross-piece may be seized with one pair 
of forceps and the hind cross-piece with another. On 
slightly pulling the two, so as to imitate the action of the 
muscles, the three teeth will be found to come together 
sharply, exactly in the manner described. 
Works on mechanics are full of contrivances for the 
conversion of motion; but it would, perhaps, be difficult to 
discover among these a prettier solution of the problem ; 
given a straight pull, how to convert it into three simul- 
taneous convergent movements of as many points. 
What I have called the filter is constructed mainly out 
of the chitinous lining of the pyloric chamber. The aper: 
ture of communication between this and the cardiac 
chamber, already narrow, on account of the constriction of 
the walls of the stomach at this point, is bounded at the 
sides by two folds; while, from below, a conical tongue- 
shaped process (figs. 6, 10, and 11, cpv), the surface of 
which is covered with hairs, further obstructs the opening. 
In the posterior half of the pyloric chamber, its side walls 
are, as it were, pushed in; and, above, they so nearly meet 
in the middle line, that a mere vertical chink is left be- 
tween them ; while even this is crossed by hairs set upon 
the two surfaces. In its lower half, however, each side 
wall curves outwards, and forms a cushion-shaped surface 
(fig. 10, cs) which looks downwards and inwards. If the 
