STRONGYLOIDES 77 



Males have not been found, according to most ob- 

 servers, but Smith claims to have found adult males. 

 His observation has not been confirmed. Possibly in 

 this generation parthenogenetic reproduction takes place. 



The free-living generation is different in all characters. 

 Both male and female forms are known. 



The females are i mm. in length and 0*05 mm. in 

 breadth. The mouth has four lips and a short cesophagus 

 with a double dilatation. Anus sub-terminal. The tail 

 end is straight and pointed. The eggs are thin shelled, 

 0*07 mm. in length and 0*045 mm. in breadth. 



The males are smaller, 0*7 mm. in length and 0*045 mm - 

 in breadth. The tail end is coiled up, the. cloaca is sub- 

 terminal, and the two spicules are equal, small, and much 

 curved. 



The life-history is of special interest. The parasitic 

 form deposits its eggs in the intestinal mucous mem- 

 brane, or in the crypts in it. The eggs develop into 

 larvae which increase in size and pass into the lumen of 

 the intestines, and leave their host in the faeces. At this 

 stage they show the double dilatation of the cesophagus. 

 In the faeces, if the temperature of the air is high, i6C. 

 or more, they rapidly develop, cast their skins and 

 become sexually mature. 



The free-living forms, male and female, copulate, and 

 each female passes thirty to forty eggs from which 

 larvae are hatched. The larvae at first are rhabditi- 

 form with the double dilatation of the cesophagus, but 

 later this is lost. The cesophagus is then simple, and 

 the larvae are filariform. They increase in size and 

 when 0*55 mm. in length moult and acquire the char- 

 acters of their grandparents the parasitic form. The 

 duration of life in this stage is about eight days, and 

 the larvae of the human species are incapable of repro- 

 duction a second time as free-living forms, and die 

 out unless they are introduced into the human intes- 

 tine, when they again become parasitic like their 

 grandparents. 



When these embryos were first discovered they were 



