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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



and anterior in the others (C, D), is flexible because the body wall 

 immediately external to it is weak, and because the two end walls 

 of the atrial chamber are membranous. The posterior or ventral end 

 of the movable wall is produced into a long, free manubrium (fig. 

 54 B, D, q) that projects into the body cavity and gives attachment 

 to two muscles, one dorsally, the other ventrally. These muscles, 

 acting antagonistically, either close or open the passage from the 

 atrium into the trachea (fig. 53 B) by means of their attachments 

 on the movable wall of the atrium. 



The short dorsal muscle of the first abdominal spiracle (fig. 54 B, 

 CMcl) arises on the rim of the tympanum {p) above the spiracle; 



Atr 



BW 



-BW 



Atr 



-u 



Tra 



Fig. 53. — Sections of spiracles of Dissosteira. 



A, longitudinal section through second thoracic spiracle, showing anterior and 

 posterior lips (c, d) as projecting folds of body wall (BIV) inclosing an atrium 

 (Atr), or entrance to trachea (Tra). 



B, vertical section through first abdominal spiracle, showing direct inflection 

 of body wall to form atrial chamber (Atr), of which anterior wall (s) is mov- 

 able, and posterior wall (0 immovable. 



Atr, atrium; BW, body wall; c, anterior lip of spiracle; d, posterior lip of 

 spiracle ; inb, membrane surrounding lips of spiracle ; q, manubrium or muscle 

 process of ventral wall of atrium ; r, intersegmental fold ; s, ventral wall of 

 atrium; t, dorsal wall of atrium; Tra, trachea; u, plate in tergal wall support- 

 ing dorsal wall of atrium. 



the long, slender ventral muscle (^OMcl) arises ventrally on an in- 

 flection of the integument mesad of the hind coxa and posterior to the 

 triangular coxal plate of the metasternum (fig. 30 A, t). It is easy 

 to demonstrate that the dorsal muscle (fig. 54 B, CMcl) is the closer 

 of the spiracle and the ventral one (Omd) the opener. A dorsal pull 

 upon the manubrium (q) of the movable ventral wall of the atrium 

 (.?) brings the inner edge of the latter against the inner edge of the 

 fixed dorsal wall (0 and thus closes the passage from the atrium 

 into the spiracular trachea. By a counter movement the passage 

 is opened. 



The mechanism of the other abdominal spiracles is the same as that 

 of the first. The short, fan-shaped occlusor muscle (fig. 54 C, D, 



