208 CALIFORNIA ACADEMY OF SCIENCES. 



This is the outer of the two tubular structures to be 

 observed by dissection alone and it is surrounded by 

 the transverse muscle-fibers (plate xv, figs. 2, 3, 4, ^i 

 and h). This is probably what Grosse referred to as a 

 tube open at both ends. 



In the anterior part of the eighth segment the walls 

 of the inner tube diverge from those of the outer one, 

 leaving a free space between them connected with the 

 exterior. A short distance back of where the two 

 diverge the walls of the inner tube are bent backward 

 and then again forward, forming a free cylindrical fold 

 projecting backward. This extends posteriorly to the 

 middle of the last segment and then ends by being 

 invaginated and forming within itself a third tube 

 (plate XV, figs. 2, 3, 4, is). The posterior part of the 

 walls of these two are fused for a short distance and 

 closely united as far forward as the fold in the second. 

 Here the innermost tube becomes entirely free and 

 lies as within a sheath surrounded by the first and sec- 

 ond tubes. This is the inner tube so easily determined 

 by dissection; it runs forward as before described, 

 passing out of the cavity between the other tubes and 

 entering the longitudinal muscles (plate xv, figs. 2 and 

 5 ^3, and plate xiii, fig. 10 int). The chitinous rod 

 (plate XV, figs. 2-5, r, and plate xiii, fig. 10, 7-) is 

 attached to the fused part of the second and third tubes 

 back of the posterior fold of the former. Only its ante- 

 rior end is shown in median longitudinal sections. It 

 is the homolog of the internal rods or plates of the 

 chitinous genitalia already described. 



It is readily seen that the intromittent organ of 3Ien- 

 opon titan is simply an exaggeration of the type of that 

 found in other species. It consists first of a deep 

 invagination of the body-wall, extending from the 



