ORIGIN OF PARASITES IQ 



we may trustfully leave the completion of the whole to the 

 future, without fear that any essential alterations will take place. 



The deductions to be drawn are as follows : That the 

 helminthes like the ectoparasites multiply by sexual processes, 

 that the entire course of development of the helminthes is 

 never gone through in the same host as is the case with several 

 ectoparasites, that the progeny at an earlier or later stage of 

 development, as eggs, embryos or larvae, quit the host inhabited 

 by the older generation, and almost always attain the outer 

 world : only in Trichinella does the development take place directly 

 in the definite host. Where the eggs have not yet developed they go 

 through the embryonic evolution in the outer world, and the 

 young larvae are transmitted, either still enclosed within the 

 egg or embryonic covering, into the intermediary host more 

 rarely they are transferred straight into the final host or they 

 hatch out from their envelopes, and after a longer or shorter period 

 of free life, during which they may partake of food and grow, 

 they, as before, penetrate, usually in an active way, into an 

 intermediary host, or at once invade the final host. Exception- 

 ally (Rhabdonema), during the free life there may be a propaga- 

 tion of the parasitic generation, and in this case only the 

 succeeding generation again becomes parasitic, and then at 

 once reaches its final host. The young forms which have invaded 

 the final host become mature in the latter, .or after a longer or 

 shorter period of parasitism again wander forth (as the (Es- 

 trides, Ichneumonides, &c.), and reach the adult stage in the 

 outer world. The young stages, during which they undergo 

 metamorphoses or are even capable of producing one or several 

 intermediate generations, are passed in the intermediary hosts 

 until, as a rule, they are passively carried into the final host 

 and there complete their cycle of development with the formation 

 of the organs of generation. This manner of development, the 

 spending of life in two different kinds of animals (intermediary 

 and final host), is typical of the helminthes. This is manifested 

 in the acanthocephala, the cestodes, the majority of the endo- 

 parasitic trematodes, a number of the nematodes, and the 

 linguatulidae ; there are now and then exceptions, however, in 

 which, for instance, the host and intermediary host exchange order 

 (Trichinella, Tcenia murina). 



Parasites are hardly ever inherited amongst animals. According 

 to a few statements, however, Trichinella and Ccenurus are sup- 

 posed to be transmissible from the infected mother to the foetus ; 



