THE EXTERNAL GENERATIVE ORGANS. 735 



podite of one or other of the lateral appendages, and in the 

 males accessory organs are frequently formed by the modifica- 

 tion of a pair of abdominal feet, which serve to direct the 

 everted genital ducts. In the Chilognatha, amongst the Myria- 

 poda, the genital apertures in both sexes are situated on the 

 basal joints of one or other of the anterior pairs of feet, 

 usually on the third pair. 



In Insects the existence of this primitive form of external 

 generative organs is exceptional ; but Palmen [332] has shown 

 that in Labidura and in the Ephemeridse, in the larva and 

 pupa, the orifices of the genital ducts are paired, and are 

 usually situated on or in relation with papillae. The union of 

 these papillae in the median line, and the formation of an azygos 

 ejaculatory duct, appears to be the advanced form, and it is 

 probable, I think, that the median external structures which 

 result from this advance are the result of the partial fusion of 

 the corresponding abdominal appendages. 



In some Insects the penis consists mainly, if not entirely, 

 of an eversible ejaculatory duct, which is placed between the 

 anterior and posterior gonapophyses. The latter, which are 

 dorsal to it, are frequently very complex, and form an arma- 

 ture connected with a permanently everted duct or a papilla 

 corresponding to the orifice of the ejaculatory duct. When 

 the posterior gonapophyses assume this form they support the 

 penis, and consist of two rami, so that they correspond with 

 the canula and lancets of the sting of the aculeate Hymen- 

 optera, and with the upper blade of the ovipositor of the 

 Orthoptera and Hymenoptera. 



In many Insects the penis possesses a very rudimentary 

 armature, but is withdrawn within a membranous sheath, the 

 so-called prepuce of the earlier writers. In those forms in 

 which these parts are very large, there are frequently no traces 

 of the somites behind the progenital ; but when, however, the 

 prepuce is large, and the penis comparatively small, the 

 genital armature is usually highly developed, and the prepuce 

 forms the genital pouch or sinus, within which the gonapo- 

 physes are retracted. 



48 



