No. 6.] STRUCTURES OF THE ANTEXXATA. 28 1 



erate group such as the Strepsiptera. The primarily segmental 

 Anlage, which made possible the varying conditions in the gen- 

 ital aperture, persists here in the development of ectodermal 

 structures in four segments instead of only one. 



The embryonic development throws further light on the 

 orio-in of these structures. The conditions in Xiphidmin and 

 OccantJins have already been described and figured. In the 

 paper cited the abdominal invaginations are said to disappear, 

 together with the rudiments of the abdominal appendages. 

 And in a XipJddium larva 3.5 mm. long the invaginations are 

 seen for the most part to have grown much shallower, but the 

 one lying just behind the ninth segment has grown only a little 

 shallower and much broader, and has come to lie in close rela- 

 tion to the terminal ampullae of the genital ducts. This, then, 

 is the pocket-like invagination, which later breaks through into 

 the mesodermal ducts and becomes the ductus ejaculatorius. 

 And we have here apparently the direct passing over of one 

 of the segmental invaginations into the terminal ectodermal 

 portion of the reproductive system. 



We have, therefore, traced both the apodemes and the ecto- 

 dermal part of the sexual ducts to a primitive condition, which 

 is a mere median deepening of the intersegmental fold, arising 

 in connection with the segmental appendages. This segmental 

 arrangement of median ingrowths and appendages for all the 

 body segments often occurs in the embryonic development of 

 the higher Antennata, but the ingrowths for the most part 

 disappear along with the ephemeral appendages. Only in the 

 thorax, and here and there in the abdomen, they persist, become 

 filled or lined with chitin, and, being brought into relation with 

 the muscular or the reproductive system, serve as the apodemes 

 or as the terminal ectodermal portions of the reproductive ducts 

 in both sexes. 



