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WHEELER. [Vol. VIII. 



both sexes the orifice lies at the posterior edge of the fifth 

 abdominal segment. In the Thysanura the female opening 

 usually lies on the eighth, and that of the male on the ninth 

 abdominal segment. In female Orthoptera and Ephemeridea 

 the sexual organs open behind the seventh, in the male behind 

 the ninth abdominal segment, while in the Plecoptera and 

 many other insects the female orifice is said to lie at the pos- 

 terior edge of the eighth segment. Although these facts of 

 adult anatomy point to great instability in the segmental ter- 

 mination of the sexual ducts, the evidence from embryology is 

 more conclusive. The female Xiphidmm embryo has at first 

 two pairs of ducts, and in the male the single pair shift their 

 position from the tenth to the ninth segment. The former 

 fact proves conclusively that the male and female ducts are 

 not homologous but homodynamous structures, and the latter 

 that ducts may shift their insertions from one segment to 

 another during ontogeny. The inference is, that sexual ducts 

 may arise in any nephridium-bearing segment from the pair 

 of nephridia which best subserve the sexual function and at 

 the same time interfere least with the development and func- 

 tion of other organs, and that a phylogenetic shifting of the 

 ducts has probably taken place repeatedly. The position of 

 the genital ostia on a particular segment cannot therefore be 

 regarded as a character of high morphological value, at least 

 for the larger groups. 



Two conflicting views have long been entertained respecting 

 the morphological significance of the gonapophyses. Under 

 this term, introduced by Huxley ('77), we may include the ap- 

 pendages of the eighth to the tenth abdominal segments in the 

 female and such of their homologues as persist in the male. 

 In the female, these appendages go to form the ovipositor. 

 According to Lacaze-Duthiers ('49-'53) they are not true append- 

 ages, i. e. homodynamous with the legs, mouth-parts, etc., but 

 simply modified ventral sclerites. Haase ('89), too, believes that 

 the gonapophyses are not true appendages but '' Integument- 

 bildungen von etwas hoherer Werthigkeit als die Griff el," or 

 styloid processes which are found inserted at the bases of the 

 legs in some Myriopods and Thysanura. A similar view ap- 



