182 OXYURIS-LARViE HATCHED IN THE HUMAN STOMACH, 



it is usual in R h a b d i t i s. The pharynx is open, though it is 

 already less like the pharynx of R h a b d i t i s than it was during 

 the tadpole stage, when it was deeper and cylindroid, consti- 

 tuting one of the striking features of the embryo. The nerve- 

 ring has not the position usual in Rhabditis (behind the 

 median swelling), but approaches it much more nearly than does 

 that of the adult O x y u r i s. Finally, the difference between the 

 sexes which already exists in the larval state * points to a deep- 

 rooted tendency toward an early degeneration of, the male tail, 

 something which finds a ready explanation in supposing the sexes 

 to have been unlike in the ancestors of Oxyuris. Now the 

 sexes are more unlike in R h a b d i t i s than iu most free- 

 living genera, but its larvct do 7iot show so early or so striking 

 a sexual differentiation as is to be seen in Oxyuris. 



If these considerations are as weighty as they seem (to me) to 

 be, we are justified in deriving Oxyuris directly from some 

 part of the group of Rhabditis-like free-living forms. 



Co7itagion. 



The world-wide common occurrence of Oxyuris v e r m i- 

 c u 1 a r i s points to an efiicient method of distribution. The 

 method is simplicity itself. The 20,000 eggs which at a moderate 

 calculation each adult female contains are laid either in the large 

 intestine to pass out with the excrement or are laid just outside 

 the anus. They become distributed over the person, clothing, bed- 

 clothing and other ai-ticles of furniture, of the infected, and in 

 excrement are sown broadcast over cultivated lands. They then 

 find their way to the mouth, mostly by way of the hands, less 

 commonly by other agencies, are swallowed, hatch promptly in the 

 stomach,! and on reaching their proper habitat, the colon, are pre- 

 pared to hold their place there until maturity.. 



The abundance of the eggs about dwellings, &c., is difficult to 



* True also of the larvae of some Oxy uri ds inhabitmg insects. See 

 Caleb, I.e. 



t This link in the history was first established by the experiments just 

 described. 



