SURVEY OF INVERTEBRATES 75 



The situation is complicated still further by the fact 

 that different stages in the life cycle of one and the same 

 worm may occur under conditions differing widely with 

 respect to oxygen content. For instance the parasitic 

 generation of Strongyloides lives in the intestine, where- 

 as the larvae normally undergo their further development 

 in the outside world. In the case of so-called heterogen- 

 etic development an entire free-living generation inter- 

 venes before the offspring returns to parasitic life. 



It can be expected that such differences will be re- 

 flected in the metabolic processes, but unfortunately rel- 

 atively few species have been investigated thus far. The 

 available information concerns chiefly intestinal para- 

 sites, fewer data are at hand for tissue helmiiitlis, and 

 practically nothing is known about the metabolism of 

 parasitic worms that live in environments rich in oxygen. 



The behaviour and metabolism of parasitic helminths 

 can be studied under controlled conditions only if they 

 are maintained outside their hosts. A great difficulty, 

 quite generally encountered in these investigations, is 

 that most helminths remain alive only for very limited pe- 

 riods when removed from their normal habitat. How- 

 ever, in recent years promising results have been ob- 

 tained in lengthening the survival period in vitro, or 

 even in rearing the worms in true culture. Glaser and 

 Stoll (1938) succeeded in obtaining a sterile culture of 

 the free-living stages of Haemonclms contortus. Glaser 

 (1940, 1940a) carried Neoaplectana glaseri, a nematode 

 parasite of the Japanese beetle, through its entire life 

 cycle in culture. Ferguson (1940, 1943) obtained the de- 

 velopment of the metacercariae of PostJiodiplostomiim 

 miimnum into adults and of the cercariae of Diplostomum 

 flexicaudum into metacercariae ifi vitro, Hoeppli and 

 Chu (1937) maintained Clonorchis sinensis alive up to 5 

 months, Lee and Chu (1935) Schistosoma japonicum up 

 to 2V2 months, von Brand and Simpson (1942) the larva 



