536 COMPARATIVE ANATOMY chap. 



shortly before entering the common duct. Before the latter enters the penis, its 

 wall becomes strongly muscular. This muscular portion of the duct evidently serves 

 as a propelling organ for driving the semen out of the penis. A pair of accessory 

 glands enters the end of the penis sheath. 



In the female the common efferent duct has 2 divisions, the proximal division 

 being enlarged into a uterus, which at maturity is filled with eggs, the narrow and 

 long distal 2iortion being a vagina which is continued into the ovipositor. The vagina 

 has 3 lateral sacs, which are regarded as receptacula seminis. Accessory glands enter 

 the end of the sheath of the ovipositor. 



The genital aperture in both sexes lies ventrally, on the boundary between the 

 ceplialo-thorax and the abdomen. 



It often occurs in the male Phalangidce that eggs develop on the surface of the 

 testes ; these apparently do not leave the body, but are reabsorbed. 



Cyphophthalmidse. — Here also the genital aperture lies ventrally at the base of 

 the abdomen (on the first abdominal segment). The male has a long penis, the 

 female a long ovipositor. 



Acarina (Fig. 375, C, G, S, I, Fig. 376, F, G, I). — Great variety here prevails in 

 the structui'e of the sexual organs. The following are 2 extreme cases. In the 

 first there are 2 separate symmetrically-placed germ glands and 2 separate ducts, 

 opening outwards through a common unpaired copulatory organ. We here see the 

 more original arrangement. The other extreme is rare. It is found in the female of 

 the Gainasidce (Fig. 375, S), where a single unpaired ovary is continued into a single 

 unpaired duct, which opens outwards through the copulatory organ. Transition 

 forms between these 2 extremes are very commonly found. The 2 germ glands fuse 

 in various ways to form one, which sometimes still shows traces of its originally 

 double character ; the ducts, however, remain separate to a greater or smaller extent. 



Accessory organs, glands, receptacula seminis are often connected with the ducts. 

 The unpaired terminal portion of the ducts nearly always leads to the outer sexual 

 apparatus, which in the male is the penis, and in the female may be developed as an 

 ovipositor. There are often found near the genital apertures adaptations {e.g. suckers) 

 which assist in copulation. The sexual organs are by no means limited in position 

 to the posterior part of the body, on the contrary, the fact that they often run far 

 forward shows the extent to which the concentration of the whole body and the 

 obliteration of the boundary between the cephalo - thorax and the abdomen have 

 taken place. The aperture of these organs often lies far forward also, in some cases 

 as far forward as between the most anterior pair of legs. It has been observed in 

 the Tyroglypha {Trichodactylus anonymus) that the genital aperture which in the 

 adult female lies between the 2d pair of legs, in the last larval stage (before the 

 last moult) still lies between the last pair. This observation also throws light on 

 the anterior position of the stigmata in many mites, which must be attributed to 

 displacement. 



As in the Araneidce, so also in certain Acciridm there occur in the females recepta- 

 cula seminis with apertures separate from the rest of the sexual apparatus. Tricho- 

 dactylus thus has a receptaculum at the posterior end of the body, opening outward 

 through a post-anal aperture. The penis is introduced into this aperture during 

 copulation. The receptaculum is connected by 2 short tubes with the 2 ovaries. 

 This an-angement and that found in Epeira recall to a certain extent the well-known 

 arrangement in the Tremaloda and Cestoda, where the female sexual apparatus is 

 connected with the exterior not only by means of the usual genital aperture, but by 

 Laurer's duct as well. In Trichodactylus the receptaculum arises independently by 

 an invagination of the integument, and becomes connected with the ovaries only 

 secondarily. 



Some Acarina are viviparous, others ovoviviparous, i.e. the eggs develop to a 



