HYPOBIOSIS IN PARASITIC WORMS 99 



In most cases, ova predestined to be swallowed are thick- 

 shelled and resistant to environmental factors. They may 

 survive during long periods of quiescence. The egg of the 

 common human parasite Ascaris serves as an outstanding 

 example of such resistance ; it can withstand most of the usual 

 antiseptics and may remain alive and infective for as long as 

 eight years in 4 % formalin. 



FREE LARVAE 



The hatching of larva depends on both external factors 

 (temperature, enzymes of the host, etc.) and the internal 

 mechanisms of the larva (pressure, specific enzymes dissolving 

 the shell from within, etc.) activated by external factors. An 

 interesting phenomenon occurs in the case of eggs of a plant 

 nematode of the genus Meloidogyne (= Heterodera). They may 

 remain quiescent in the ground for a long period and are 

 stimulated to hatch by some substances produced and excreted 

 by growing plant roots^. 



In many parasitic worms the infective larvae live for a varying 

 period outside their host, as in the case of the miracidium and 

 cercaria of trematodes, of the coracidiiim of cestodes or several 

 stages of the developing larva of nematodes. However, these 

 stages, except the early nematode larvae, are not fully active 

 physiologically. Miracidia, cercariae and coracidia do not feed 

 or develop, they move about in order to reach a suitable host. 

 This is a period during which biological activity is limited in 

 function and time and it may be regarded as a kind of hypo- 

 biosis. Yet some of the nematode larvae, notably of the family 

 Strongylidae of horses (so-called red worms) are remarkably 

 vital, being able to live on the pasture twelve months or even 

 longer, without change provided climatic conditions are suitable. 



In some parasitic nematodes the infective larvae crawl onto 

 grasses and remain on them until they are eventually picked up 

 by grazing animals'^. In some cases these larvae may slowly dry 

 up intil they become fragile, surviving for long periods in this 



References p. 106 



