HYPOBIOSIS IN PARASITIC WORMS 



G. WITENBERG 



Department of Parasitology, Hebrew University, Hadassah Medical School, 



Jerusalem (Israel) 



Numerous examples of temporary cessation of vital physiolo- 

 gical processes are known to occur at various stages in the life 

 of parasitic worms. For the most part, empirical observations 

 have been recorded without any attempt to explain their 

 physiological basis. True diapause, which is characterised by a 

 state of obligatory rest for a certain prolonged period, apparently 

 does not occur in parasitic worms. Cases of curtailment or 

 cessation of activity must be regarded as phenomena of adap- 

 tation which allow the egg or the larva to perform the stage 

 transformation without disturbance, or the adult to survive 

 unfavourable environmental conditions. This kind of crypto- 

 biosis^ has usually no predetermined limit and may end as soon 

 as conditions become suitable. 



EGGS 



Eggs of helminths are laid in various stages of development, 

 depending on the species. In the case of undeveloped eggs, 

 development may start only after they have been evacuated 

 from the host and have come under the influence of atmospheric 

 oxygen, given suitable conditions of temperature and other 

 climatic factors, as for instance, ova of the most common human 

 parasites Ascaris and TrichocephaJus. If the eggs are introduced 

 into an environment lacking the necessary climatic conditions or 

 free oxygen (for instance, at the bottom of fermenting sewage 

 tanks) embryonic development is retarded or stops. It will 

 usually be resumed when proper conditions are restored. Not all 

 developmental stages of the embryo are equally adapted to such 

 interruptions; generally, more advanced embryos are better able 

 to withstand them. 



Capillar ia hepatica, a nematode living in the Uver-parenchyma 



References p. 106 



