280 .1 A - 1 /Jh'OIUOSIi^ A XD EMH)r. 1 A'. 1 ^ITI^M 



cease to feed when the oxygen is removed, while the 

 intestinal parasites are then still able to assimilate food. 

 A third objection is that it is questionable whether nmd, 

 considered from a nutritional standpoint, would serve 

 as a good "Vorschule" for a future establishment in the 

 intestinal tract. Mud is usually very poor in nutritive 

 material utilizable by metazoa, while the intestine has 

 an abundant supply of it. 



The present writer considers Alsterberg's first and 

 third points as rather weighty, but cannot regard the 

 second argument as conclusive. The facts reviewed pre- 

 viously indicate that both in free-living animals and in 

 parasites marked differences in the degree of tolerance 

 towards the lack of oxygen are encountered; but these 

 differences seem to be only quantitative, not qualitative. 



There are still other objections to Bunge's views. The 

 lack of oxygen is not the only adverse condition that in- 

 testinal parasites have to contend with. They must also 

 resist the action of the digestive enzymes of their hosts, 

 be it through the development of anti-enzymes, or other- 

 wise. They must be able to live in an environment of rel- 

 atively high osmotic pressure, many must be quite inde- 

 pendent of the hydrogen ion concentration, etc. It is not 

 easy to understand how all these prerequisites could have 

 been acquired in the mud. 



There are other possibilities. Pintner (1922) visual- 

 izes a gradual phylogenetic development of the parasitism 

 of worms by the following steps : ecto-parasitism -par- 

 asitism in mesenchyme > in lymph > in blood 



' within the respiratory tract > within the 



intestinal tract. 



While this scheme obviously does not require a mud-an- 

 cestry, it is not too satisfactory from the viewpoint of 

 gradual adaptation to anaerobic conditions. The passage 

 from the respiratory tract to the intestine must involve 

 quite abrupt changes in oxygen sui)ply ; a gradual acqui- 



