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 Jllter 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 



