20 THE INVERT^BRATA 



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

 Q%g 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 contraction 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 

 they 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 

 fitting skin as a so-called "encysted" larva which possesses, however, 

 the power of movement. The protection of the cast skin and possibly 

 other factors enables this stage in the life history to resist desiccation 

 and to remain in a state of dormant metabolism until some odour of 

 decaying substances attracts the larvae and the opportunity of rapid 

 reproduction is given for a brief period. 



This third larval period is characteristically the period of wander- 

 ing in many nematodes, and this is seen in a remarkable manner in 



