78 



LIFE-HISTOEY OF PARASITES. 



Gordiaceae and Mermithidae are instances of this kind of parasitism 

 among the Entozoa, and the migration from the body of their host of 



Fio. 55. — Lung of rabbit infected 

 with Pentastomida. 



Fig. 56. — Pentastomum denticulatum. 



proglottides and other sexual Helminths (e.g., Oxyuris vermicularis) 

 presents an approximation to the same phenomenon. 



The young of these periodic parasites, at least in the case of 

 insects, show certain peculiarities induced by the fact that their 

 migration into the body of a host is accomplished for them by their 

 parents. The latter, possessing as they do the power of free locomo- 

 tion, can evidently influence considerably the fate of their eggs, which 

 is quite as evidently impossible to the Entozoa. Thus the gad-flies lay 

 their eggs on the hair of certain mammals, in situations whence the 

 young can easily in an active or passive manner (e.g., by being licked 

 up) reach their next destination. The Ichneumonidse make matters 

 easier still for their descendants, by depositing their eggs directly in 

 the perivisceral cavity of caterpillars, for which purpose they are 

 provided with a suitably constructed boring ovipositor. 



The converse of this is illustrated in the Gordiacese and Mermithidse, 

 whose eggs are laid in water or damp earth, and the young when hatched 

 find their own way by active migration into their proper host, as has 

 already been said. Whether the embryo be conveyed passively or 

 actively, it makes its way into the body of its host, and becomes in 



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