40 CLAUDE FULLER. 



a stiffened rim which is inserted into the cuticle of the body- 

 wall. 



(2) The atrium ; an elongate and wrinkled cylindrical tube 

 distended at the pore by a more or less regular tessellated 

 framework of a hexagonal pattern ; the approximation of 

 the strands forming the heavier chitinisation of the rim of 

 the pore, fig. 37 (PL V). At the end of the atrium, on the 

 visceral side, is a small plate (2a) which is the "resistor." 

 The atrium seems to lead directly into the ascidiuni of the 

 lever. 



(3) The lever ; this lies exterior to the trachea and has 

 every appearance of being a simple membranous sac ; but, as 

 it is rigid, only its visceral side can be membranous. 



(4) The epi-trachelos ; a small membranous tube whose 

 mouth surrounds the opening between atrium and lever and 

 through which the air passes into the trachelos. It is the 

 mouth of the epi-trachelos which is closed by the bending of 

 the lever towards and across the resistor, fig. 23c. 



(5) The trachelos is the hexagonally tessellated and bulbous 

 foot of the tracheal pipe. 



The developments which first take place are more or less 

 concurrent. The lever gradually loses its vesicular form and 

 so changes to a plate which is slightly concave on the visceral 

 side and convex on the external side. Figs. 25a, b, c repre- 

 sent the lever in vertical section and show what appears to 

 take place. The resistor of the atrium develops a deep hiatus, 

 the hiatus arising from the ingrowing of the lower median 

 region, which now protrudes into the lumen, figs. 26a and b. 

 The region of the atrium adjacent to and anterior to the 

 resister becomes prominent and chitinised, forming a " chin- 

 plate " (c-/(., fig. 30). The epi-trachelos is absorbed into the 

 trachelos which extends and reaches to the opening between 

 atrium and lever. These changes produce the spiracle of the 

 second stage wdth its quoit-like rim, figs. 24a and b. 



The opening and closing of the lumen is brought about by 

 a band of muscle. This has three fixed insertions, the first 

 is to the chin-plate {ch., figs. 28a and 30) slightly anterior to 



