250 THE INVERTEBRATA 



the head and the tail. The ovary is a short syncytial tube, the nuclei 

 becoming larger and larger and the centre of more definite and larger 

 aggregations of cytoplasm and yolk nearer the uterus. Finally, there 

 is a single ovum discharged at a time into the oviduct ; as soon as this 

 happens another ripens in its place. To reach the uterus the egg has 

 first to pass through a portion of the oviduct {receptaculum seminis) 

 filled with the amoeboid spermatozoa of the male. Fertilization takes 

 place, a shell is formed and at the same time maturation proceeds. 

 The two uteri join to form the median vagina. In this the fertilized 

 egg develops and the young larva is formed and may hatch within the 

 vagina. The stages of segmentation are seen nowhere with such ease 

 or clearness as in a small transparent nematode of this kind. 



The male, on the other hand, has only a single gonad. The apical 

 testis is syncytial like the ovary. Nearing the vas deferens a zone may 

 be seen of free spermatocytes and in the vas deferens itself can be seen 

 large numbers of rounded spermatozoa. The genital duct opens into 

 the gut to form a cloaca. This contains a dorsal pocket in which is 

 secreted a chitinous apparatus consisting of two converging rods, the 

 copulatory spicules ^ with a grooved connecting piece to hold the points 

 together. The pocket has a special muscle which protrudes the spicules 

 from the anus (cloacal aperture). To each side of this aperture is a 

 lateral cuticular flange, supported by ribs, which meets its fellow at 

 the root of the drawn-out tail. This acts as a sucker (copulatory bursa) ^ 

 by which the male retains its position on the body of the female until 

 the spicules are thrust through the female aperture and keep the 

 female and male apertures both apposed and open. Then by the con- 

 traction of the muscles of the cloaca the spermatozoa are expelled and 

 passed into the vagina of the female. Here they become amoeboid 

 and travel up the uteri so that they can meet the ova as the latter 

 are discharged. 



Besides the normal condition in which males and females are pro- 

 duced in equal numbers, many species of Rhabditis occur in which 

 there is a remarkable disparity in numbers of the sexes. For a thou- 

 sand females there may be only ten or twenty males, and they are 

 lethargic in their sexual activities. The females, on the other hand, 

 have developed a curious kind of hermaphroditism. When the gonad 

 first becomes ripe a number of spermatozoa are produced. Afterwards 

 the gonad produces nothing but eggs which are fertilized by the in- 

 dividual's own spermatozoa, and after these are exhausted nothing 

 but sterile eggs are laid. Experiment has proved that in these animals 

 self-fertilization may occur for an immense number of generations 

 without any deterioration of the species. 



In Rhabditis, as in the majority of nematodes, there are four moults. 

 After the second moult the animal may remain within the loosely 



