THE ORGANS OF CIRCULATION AND RESPIRATION. 



153 



seen from within, and shows that the slit divides the cup into 

 two unequal lips, the smaller of which inclines away from the 

 middle line of the body, is movable, and is strengthened on its 

 deep surface by a curved chitinous rod, the "bow' of Landois. 



Fig. 80. Second Thoracic Spiracle (left side), seen from the outside. X 70. V, lower 

 (movable) valve. The occlusor muscle is shown. The arrow indicates the 

 direction of air entering the spiracle. (P. americana.) 



From the opposite lip, a pouch is thrown out, which serves for 

 the attachment of the occlusor muscle. The muscle is inserted 

 into the extremity of the bow, and when it contracts, the bow is 

 pulled over into the position shown in fig. S7D, and the opening 

 is closed. The antagonist muscle, which exists in all the 

 abdominal spiracles, is shown in fig. 88 ; it arises from the 



