NO. 14 INSECT ABDOMEN SNODGRASS 33 



The external genital chamber, or bursa genitalis (fig. 4G-I, GC), 

 commonly known in arachnology as the " uterus externus ", is un- 

 doubtedly an ectodermal pouch of the body wall since it always has 

 a thick cuticular lining. Into its anterior end open either the paired 

 gonoducts (G), or the median ductus conjunctus (H) or saccus inter- 

 nus (I). Diverticula of the bursal wall form accessory genital glands 

 of various kinds, often of large size, and other structures of unknown 

 function, which in some cases are eversible. In the female a pair of 

 lateral pouches may serve as seminal receptacles. The external open- 

 ing of the genital chamber, or gonotreme (Gtr), as already observed, 

 is always associated with the venter of the second opisthosomal seg- 

 ment, which is segment VIII (figs. 10 D, E, 11 A, D, 13 A, Gtr). 

 In the more generalized condition the gonotreme lies behind the ster- 

 nal plate of the genital segment (figs. 10 E, 11 A, B), but when the 

 genital region is displaced forward between the legs, as in the Pha- 

 langida (fig. 13 A), the genital sternum may become reduced or con- 

 cealed by invagination within the bursa. 



A comparison of the external genital structure of the more general- 

 ized Arachnida with that of Xiphosura shows that the lumen of the 

 genital chamber in the former, into which the gonoducts open, must 

 correspond with the external cavity behind the operculum of the latter, 

 which contains the gonopores. The relation of the arachnid structure 

 to the more primitive condition in Xiphosura is particularly evident 

 in the scorpions, where the genital chamber opens behind a small 

 opercular plate or pair of plates (fig. 10 D, E, Opl) on the venter of 

 the eighth segment, and the gonoducts open separately into the anterior 

 end of the enclosed cavity. The operculum of Xiphosura, as we have 

 seen, clearly includes the appendages of the eighth postoral somite 

 and a median fold of the venter of the same segment (fig. 9 B, V), 

 on the posterior surface of which are located the genital papillae 

 (Pen). In a truly primitive stage of the Chelicerata (except Pycno- 

 gonida), therefore, the paired gonoducts of each sex must have opened 

 on the venter of the eighth postoral segment mesad of the bases of 

 the appendages. 



Most of the Arachnida do not have an organ for intromission of 

 the sperm associated directly with the gonotreme. The mating habits 

 of the arachnids are well known only in a few groups, but generally 

 if the male personally inserts the spermatozoa or spermatophores into 

 the sperm receptacle of the female, he uses for this purpose a pair of 

 the body appendages, such as the chelicerae in the Solpugida, or the 

 pedipalps in the Araneida. The female, however, may be inseminated 

 in some indirect manner, as with the Chelonethida. At the time of 



