ANAER0BI08IS A ND EN DO PAR A 8ITIS3I 281 



sition of anaerobic habits can hardly have come about in 

 that manner. 



Much more satisfactory in this respect is Stunkard's 

 (1937) view. Taking into consideration the bionomics, 

 development and morphology of the flat-worms, he de- 

 veloped the idea that Turbellaria, Mesozoa, Cestoda and 

 Digenea have ''descended concomitantly from a com- 

 mon ancestral group of planula-like ancestors." Ac- 

 cording to him, the predecessors of the present-day para- 

 sites belonging to these groups became parasites of in- 

 vertebrates in early geological times, long before they 

 invaded vertebrates. This view obviously opens new 

 angles to the question of adaptation to the extreme 

 conditions that parasites encounter in vertebrate hosts. 

 It has been indicated previously that the tissues and 

 perhaps also the intestines of many invertebrates are not 

 so poor in oxygen as those of vertebrates ; the other ad- 

 verse conditions mentioned above may likewise not be so 

 pronounced in the former as in the latter. A gradual 

 adaptation to extreme conditions would thus have oc- 

 curred through a successive invasion of various hosts. 



The idea that the ancestors of some parasitic worms 

 might have been mud-dwellers is not, however, to be dis- 

 missed altogether. The nematodes, which are extremely 

 resistant to all kinds of injurious agents, might have 

 passed from a non-parasitic life in the mud to that of 

 parasites in the intestine of vertebrates. It is of inter- 

 est in this connection to recall that Moore (1931) found 

 mud-dwelling nematodes with such pronounced anaerobic 

 functions that they appeared to be normally anaerobic. 



As to Bunge's view that ultimately the parasitic worms 

 and arthropods are derived from aerobic ancestors, it 

 seems well-founded. It is supported by the fact that all 

 species investigated so far are capable of consuming 

 oxygen when it is available. In most instances they even 

 require oxygen at some stage during their life cycles. 



