FREE-LIVING NEMATODES 473 



As the male organ may be either double or single, outstretched 

 or reflexed, the corresponding abbreviations for the usual forms of 

 male apparatus are as follows: 



-M =M 'M -M -M- 



and this is the form in which the facts are presented in the formulae 

 for males. As the testes always lie in front of the sexual opening, 

 the datum point of the reference signs in this case is the point 

 where the testes join the vas deferens, not the sexual opening, as 

 in the females. The percentage figure representing the extent 

 of the male sexual organs dates from the anus. Species with re- 

 flexed testes are comparatively rare among fresh-water nematodes, 

 the commonest forms being -M- and -M. 



The blind, free, or distal end of the female sexual tube is usually 

 found to contain only cells of extremely small size, observable with 

 difficulty. In consequence little is known about the primordial 

 sexual products in these free-living species. The interior of the 

 main part of this segment of the tube, the ovary, is filled with devel- 

 oping oocytes, which generally soon arrange themselves in single file. 

 The oocytes increase rapidly in size, so that they are ripe by the time 

 they reach the entrance to the uterus. At this point they undergo 

 synapsis, meet the spermatozoa, and are fertilized, and then receive 

 their shells, cuticular coverings acquired in the uterus. The sper- 

 matozoa usually collect together at the end of the uterus, which, in 

 some instances, has a special form adapted to their reception, and 

 in all cases must be at least physiologically adapted to attract and 

 retain them. Some species have special receptacles for the sper- 

 matozoa in the shape of large tubular branches of the uterus, 

 genuine spermathecae. 



The entrance to the uterus from the ovary is narrow, and this 

 slender part of the sexual tube is armed with delicate annular 

 muscles adapted to moving the ova on into the uterus. The 

 uterus varies much in size. Frequently in the small species a 

 single egg completely fills it; in the larger fresh- water species each 

 uterus may become large enough to carry a score or more of eggs. 

 In the larger parasitic species this capacity is enormously greater, 

 so that the number of eggs in the uterus may reach tens of thou- 

 sands, or even hundreds of thousands. 



