202 



CALIFORNIA ACADEMY OF SCIENCES. 



part ill which the operation occurs is carried backward 



Fig. 5 — Diagram of external genitalia of male aiallophaga; a, anus ; gc, genital 

 chamber; e;, ejaculatory duct; jj, penis ; p?, internal plate arising from chitinous 

 thickening in base of penis. 



as an invagination forming an internal tube arising 

 from the anterior wall of the genital chamber. This 

 tube is the penis. Figure 5 represents diagrammatic- 

 ally what has just been described. It is evident that if 

 the inner walls of the genital chamber are flexible the 

 penis could be entirely protruded from the cavity con- 

 taining it. Specimens killed in alcohol are often found 

 with the penis projecting in this manner. The anal 

 segment is lacking, so that the rectum opens into the 

 genital chamber. 



The chitinous genitalia are formed as chitinizations 

 in the walls of the inner tube. The object of this is 

 double, (1) to strengthen the walls of the penis, (2) to 

 give attachment to muscles for protruding and retract- 

 ing the penis. The latter function is apparently the 

 more important, since a contrivance for its accommoda- 

 tion is never absent, while chitinous parts strengthen- 

 ing the penis are in some cases very slightly developed 

 and in a few, such as in Menopon titan, yet to be 

 described, and in some species of Colpocephaluin, are 

 entirely absent. The structure which gives attachment 

 to the muscles is always of the form of a chitinous pro- 

 longation into the body cavity from a chitinization in 



