1 8 THE ANIMAL PARASITES OF MAN 



course, much still remains to be done. So far, we clo not even 

 know all the helminthes of man and of the domestic animals 

 in all their phases of life, and still less is known of those of 

 other animals. We are indebted to the discoveries of the 

 last fifty years for the knowledge arrived at, though compara- 

 tively few names are connected with it. The gross framework 

 is revealed, but the gaps have only been filled up here and 

 there. However, 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 rarely or 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 develop- 

 ment take place directly in the definite host. Where the eggs have 

 not yet developed they go through the embryonic evolution in the 

 outer world. The young larvae are transmitted, either still enclosed 

 within the egg or embryonic covering, to the intermediate host or 

 more rarely they are transferred straight to the final host. In other 

 cases they may hatch out from their envelopes, and after a longer or 

 shorter period of free life, during which they may partake of food and 

 grow 7 , they, as before, penetrate, usually in an active way, into an 

 intermediate host, or at once invade the final host. Exceptionally 

 (e.g., Rhabdoneina), during the free life there may be a propagation of 

 the parasitic generation, and in this case only the succeeding genera- 

 tion 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 CEstridae, Ichneumonidae, etc.), and reach the adult stage 

 in the outer world. The young stages, during which the parasites 

 undergo metamorphoses or are even capable of producing one or 

 several intermediate generations, are passed in the intermediate hosts 

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

 complete their cycle of development by the formation of the organs 

 of generation. This mode of development, the spending of life in 

 two different kinds of animals (intermediate and final host), is typical 

 of the helminthes. This is manifested in the Acanthocephala, the 

 Cestoda, the majority of the endoparasitic Trematoda, a number of 

 the Nematoda, and the Linguatulidae. There are now and then 

 exceptions, however, in which, for instance, the host and intermediate 

 host change order (Trichinella, Hymenolepis miirina). 



