THE FILTERING APPARATUS. 59 



floor of the pyloric chamber were flat, a wide triangular 

 passage would thus be left open in its lower half. But, 

 in fact, the floor rises into a ridge in the middle, while, at 

 the sides, it adapts itself to the shape of the two cushion- 

 shaped surfaces ; the result of which is that the whole 

 cavity of the posterior part of the pyloric division of the 

 stomach is reduced to a narrow three-rayed fissure. In 

 transverse section, the vertical ray of this fissure is 

 straight, while the two lateral ones are concave upwards 

 (fig. 9, E). The cushions of the side walls are covered 

 with short close-set hairs. The corresponding surfaces 

 of the floor are raised into longitudinal parallel ridges, 

 the edge of each of which is fringed with very fine hairs. 

 As everything which passes from the cardiac sac to the 

 intestine must traverse this singular apparatus, only the 

 most finely divided solid matters can escape stoppage, so 

 long as its walls are kept together. 



Finally, at the opening of the pyloric sac into the 

 intestine, the chitinous investment terminates in five 

 symmetrically arranged processes, the disposition of 

 which is such that they must play the part of valves 

 in preventing any sudden return of the contents of the 

 intestine to the stomach, while they readily allow of a 

 passage the other way. One of these valvular processes 

 is placed in the middle line above (figs. 10 and 11, r 1 ). 

 It is longer than the others and concave below. The 

 lateral processes (z; 2 ,) of which there are two on each side, 

 are triangular and flat. 



