FREE-LIVING NEMATODES 477 



no bursa is developed, nevertheless have papillae, as they have been 

 called, located according to the same general law as the ribs of the 

 bursa. {Diplogaster , Cephalobus.) 



One striking fact will be forced on the attention of the collector of 

 nematodes early in his work, and that is the comparative rarity of 

 the males. In many of the species the males have never been seen, 

 and in most species the females are from five to twenty times as 

 common as their mates. There is reason to think that in some 

 species the males are very short-Hved, and that this is the reason 

 they are so rarely seen. The males are often so much smaller than 

 the females that they are easily overlooked, or mistaken for young, 

 so that in such cases the rarity of the males may easily be over- 

 estimated. In a few species the males appear to be more common 

 than the females, at least at times. Hermaphroditism and par- 

 thenogenesis are frequent. (See p. 495.) 



As the ova approach the narrow duct leading to the uterus they 

 rapidly acquire yolk of a distinctly granular character. In the case 

 of the numerous species having reflexed ovaries, the oviduct is 

 located near the flexure, and is so small and short that it is usually 

 impossible to see it except when the organs are immature. Passing 

 through the oviduct, the ovum enters the uterus, where for a short 

 distance the cells of the uterine wall are unusually well developed, 

 apparently to furnish the material for the shells of the eggs. Here 

 too the eggs are fertilized. The proximal limit of the shell-gland 

 is often very definite. The rest of the uterus is thin-walled and 

 connects with the vagina through a narrow muscular duct, mainly 

 responsible for forcing the eggs into the outer world. The eggs 

 at the time of deposition are usually soft and phant, so that they 

 easily pass through the vulva, even when relatively large. 



The fresh-water nematodes are typical of the entire group of 

 free-Hving nematodes in that while most of them are oviparous, 

 some are ovi- viviparous and others viviparous. The eggs in most of 

 the known fresh-water species are smooth shelled. In the segmen- 

 tation the first division is a shghtly unequal one, one blastomere giv- 

 ing rise to the somatic tissues, the other to the sexual organs. 



There are various organs that have been observed in the free- 

 living nematodes whose functions are problematical, such as (i) the 



