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to the experiments. It is very suggestive that Child 

 (1919) observed that the susceptibility of Planaria doro- 

 toccphala to anaerobic conditions does indeed increase 

 with progressive starvation at least up to an inanition 

 period of four months. Child (1919a), observed also 

 that exposure to cyanide decreases the resistance of plan- 

 arians to lack of oxygen. He assumes that the two fac- 

 tors are additive in causing injury, but a real under- 

 standing of these last observations will require further 

 study. 



Our knowledge concerning the anaerobic resistance of 

 developmental stages of ivorms is rather scanty. Barron 

 (1932) found that the eggs of Nereis could be kept for 

 5 hours, and their sperm for 3 hours and 15 minutes, in 

 the complete absence of oxygen, without being damaged, 

 but the limit of endurance was not determined. After 

 restoration of aerobic conditions the eggs thus treated 

 could be fertilized by normal sperm, and the experimental 

 sperm could fertilize normal eggs. If the aerobic con- 

 ditions were not restored only the first stages of fertili- 

 zation took place, i.e., only the fertilization cone was 

 formed, but no further changes occurred during the en- 

 tire 3-hour period of observation. Nuclear changes did 

 not take place. 



2. PARASITIC SPECIES 



A. Evidence from experiments in vitro, (a) Intro- 

 ductory remarks. Endo-parasitic worms live in 

 environments that show great variations in their oxygen 

 relationships. Many helminths are found in the lumen of 

 the intestine of vertebrates, which, as discussed previous- 

 ly, is extremely poor in oxygen. Others are tissue para- 

 sites; their environment is characterized by a somewhat 

 higher, but still fairly low oxygen tension. Still others 

 occur in the blood, in the trachea, in the swim-bladder of 

 fishes or similar habitats in which an abundant oxygen 

 supply is available. 



