56 W. BALDWIN SPENCER. 



muscular mass, and as it passes backwards and upwards a 

 groove appears on the surface, the two edges of which rise 

 up so as to leave between them an open canal semicircular 

 in transverse section (figs. 62, 66, Gr.) in which opens the 

 cirrus-tube with the cirrus (C), to which reference will be 

 made later. 



The relative position of the dilator rod-sac and the opening 

 of the cirrus-sac vary slightly according to the greater or less 

 state of contraction of the latter. 



In figs. 62 and 62a an attempt is made to show the structure 

 and relationship of the different parts as gathered from a study 

 of the parts mounted whole, and of longitudinal and transverse 

 sections. At times the dilator rod-sac may be drawn lower 

 down, and its free end cut through without showing, as in 

 figs. 64 — 66, the opening of the cirrus-sac. 



The chitinous covering of the whole mass is directly con- 

 tinuous with that of the cavity in which it lies, though here 

 the chitin is very thin. The walls of the cavity consist of a 

 layer of columnar cells internally, and of a strong develop- 

 ment of muscles running obliquely and longitudinally. 



From the posterior end of the outer wall of each cavity a 

 strong band of muscle-fibres runs downwards and forwards, 

 to be attached to the ventral wall of the body just in front 

 of the genital opening (figs. 13, 14, M^.). The bands run 

 obliquely in such a way that they cross one another just 

 before their attachment, the one arising from the right dilator 

 rod-sac being attached on the left of the median line, and 

 vice versa. 



The name of dilator rod-sac is applied to this part inasmuch 

 as it contains, as described above, a somewhat shovel- shaped 

 chitinous rod which serves as an accessory dilator organ in 

 copulation. The muscles serve on contraction to force the 

 rod downwards and out of the genital opening; whilst the 

 crossing of the muscles serves to cause each rod to press 

 outwards and away from the mid-ventral line. 



{g) Cirrus-sac. — This lies on either side of the body close 

 to the mid-ventral line, and opens together with the cavity 



