SURVEY OF INVERTEBRATES 79 



tinues up to the gastrula stage and in Enterohius vermi- 

 cularis even up to the tadpole stage (Zawadowski and 

 Schalimow, 1939; Schalimow, 1931; Wendt, 1936). 



To decide whether certain hehninth larvae can com- 

 plete their development in anaerobic environments is im- 

 portant in view of the question of ' * auto-invasion ' '. Nishi- 

 gori (1928) claims to have observed the transformation 

 in the absence of oxygen of the rhabditiform larvae of 

 Strongyloides into the infective filariform larvae. Lee 

 (1930), however, was unable to confirm these results con- 

 clusively since the establishment of truly anaerobic con- 

 ditions is exceedingly difficult in the culture medium re- 

 quired by these animals. There can be no doubt that 

 auto-invasion occurs in Strongyloides infections, i.e., 

 under certain conditions, the rhabditiform larvae change 

 into the filariform stage inside the intestinal tract of the 

 host. These larvae are, however, very small and it is 

 quite possible that in view of their relatively large sur- 

 face the amount of oxygen that they can get is not negligi- 

 ble even at the low tensions prevailing in the intestine. 



In the case of the hookworm larvae, on the other 

 hand, the normal oxygen content of the intestine of the 

 host seems to be clearly insufficient to insure development. 

 According to McCoy (1930) these larvae will still hatch 

 in water containing only 0.4 cc. oxygen per liter, i.e., 

 about one fifteenth of the saturation value. Below this 

 figure the development of the eggs is greatly retarded. 



The eggs of Ascaris seem to need a still higher oxygen 

 tension for their development. According to Brown 

 (1928) the development is retarded about 30 per cent in 

 media containing 1.3 to 1.8 cc. of oxygen per liter and 

 about 50 per cent if the oxygen content is 1.1 cc. per liter 

 (23°C). Kosmin (1928) showed that the eggs of Par as- 

 caris do not develop at all at tensions of 5 mm. of mer- 

 cury, but between 10 and 80 mm. the development is pos- 

 sible, although retarded. Above 80 mm. development 

 takes place normally. 



