DEVELOPMENT OF THE NEMATODES 271 



connected with the cloaca. In each glandular sac there is a 

 chitinous rod-like formation, the spiculum. A special muscular 

 apparatus, consisting of protractors and retractors, causes the 

 movement of the spicula ; they can be projected from the cloacal 

 orifice (anus) during copulation, and when they are introduced 

 into the vagina they serve as clinging organs, perhaps also as 

 stimulatory organs. 



The male Strongyloides possess two wing-like appendices at their 

 posterior extremity ; these are projections of the body wall sup- 

 ported by so-called ribs. This formation, most unsuitably termed 

 bursa copulatrix, serves as an organ of prehension during copula- 

 tion. Some forms, moreover, carry a suctorial disc at the pos- 

 terior extremity ; in others (Trichinella), the spicula and other 

 clinging organs are absent ; they are then replaced by an evertable 

 cloaca. 



(b) DEVELOPMENT OF THE NEMATODES. 



This is not the place to describe minutely the well known 

 processes of the fertilisation and development of the ova of the 

 Nematodes. It may, however, be briefly mentioned that fertilisa- 

 tion always takes place within the uterus, and that then the eggs 

 are surrounded by a thick or thin shell, to which may be added 

 an albuminous mass, secreted by the epithelial layer of the uterus. 

 The shape of the completed eggs is characteristic of the different 

 species, and therefore a single egg often suffices to diagnose the 

 species. According to the species, the eggs may be deposited 

 sooner or later, either before or during segmentation, or with the 

 embryo perfectly developed. Only a few species are viviparous ; 

 in the other Nematodes the further development of the extruded eggs 

 takes place after various length of time in the open, in moist 

 earth, or in water. Thick shelled eggs can maintain their develop- 

 mental capacity for a long time, even after prolonged desiccation. 



Finally, a Nematode-like embryo develops, which usually lies 

 somewhat coiled up within the shell, and varies in its further 

 development according to the species to which it belongs. 



In the simplest forms, as in the free-living Nematodes, the 

 embryos, apart from their size, resemble their parents, and are 

 fully developed after leaving the egg shell. In many parasitical 

 Nematodes, however, the young must be called larvae, as they 

 present characters which are subsequently lost. 



The manner of conveyance of the larval Nematodes from the 

 O p en which they have almost always attained from the body 

 of the host into the definitive host is very different in the various 



