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 [882] has shown 
that in Labidura and in the Ephemeridz, in the larva and 
pupa, the orifices of the genital ducts are paired, and are 
usually situated on or in relation with papilla. The union of 
these papilla 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 ora 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armature, but is withdrawn within a membranous sheath, the 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 
