PAIRED SEXUAL OUTLETS 491 



and as already stated it is very marked in Limulus, where the paired 

 outlets are in both sexes very simple and wide apart (Fig. 3, A). In 

 the worms the paired genital ducts are modified segmental organs. 

 As we have seen, in the young male Lepisma there are two male 

 genital openings. Hence this double nature of the genital passages 

 in the may-flies seems to be very primitive. 



In the Dermaptera, also, the genus Labidura was found by Meinert 

 to have two independent ductus ejaculatorii, opening externally in 

 dotfble external slit-like processes (penes). The two ducts arise 

 from a single seminal vesicle, which is either paired (L. advena), or 

 forms a common passage (L. gigantea). In Forficula (Fig. 464, B} 

 only one ejaculatory duct persists, the other is obliterated, and one 

 of the penes is atrophied, the other assuming a position in the mid- 

 dle line of the body. Thus the single ejaculatory duct and seminal 

 vesicle arise from the primitive vasa deferentia, and not from the 

 integument of the body, as is the case in the following examples. 



According to the researches of Dufour, Loew, etc., most species of Orthoptera 

 (CEdipoda), Libellula, Perla, Panorpa, Khaphidia, Myrmeleon, Sialis, and Tri- 

 choptera (Hydropsyche) have double vasa deferentia and seminal vesicles, and 

 two ejaculatory ducts. The male genital passages of Rhaphidia have a double 

 opening, Loew describing "the two seminal vesicles as lying near each other and 

 at last uniting in a common passage, with an external opening, which, however, 

 must be very short, since I could only once clearly observe it." This opening 

 is a deep invagination of the external integument, at the bottom of which the 

 two ducts open independently of each other. In such insects, Palm^n states 

 that the single ejaculatory duct morphologically arises by an invagination of the 

 integument. 



In another group, forming, as regards the genital apparatus, a step next above 

 the Ephemeridse, viz. the Perlida?, the oviducts open near each other at the 

 bottom of a median single "vagina," situated between the 7th and 8th abdomi- 

 nal segment ; it is covered beneath by a valve-like, enlarged sternite of the 

 preceding segment, and Palmen homologizes it with the ovi-valvula of some 

 Ephemeridse. He regards this bell-shaped vagina as a cup-like, deep, interseg- 

 mental fold, which projects into the body-cavity and there receives the two 

 ducts. 



This differentiation in the Perlidse may be regarded as the type for several 

 groups of insects. But in others occur a complication which in some degree 

 modifies the type. Thus the invagination arises out from one segment alone, 

 but several segments during metamorphosis may become so reduced that the 

 ventral portions of all may be invaginated to form the vagina. Thus in the 

 larva of Corethra, according to Leydig, and also Weismann, the two testes are 

 attached by two cords to the integument ; the hinder ones are inserted indepen- 

 dently, and share in the development of the outlets. 



Graber has observed the same relations in the pupa of Chironomus, the efferent 

 genital tubes in both sexes being separate, so that there are two vaginal passages 

 and two penes present. Palme'n comments on these relations in the dipterous 

 insects, remarking that during metamorphosis certain parts of the terminal ab- 

 dominal segments are reduced, while others are hypertrophied ; hence the points 



