44 



LIFE-HISTOllY OF PARASITES. 



also questionable whether in this latter group parasitism is ever con- 

 fined to the female alone, as has been very generally observed to be 



the case in the Lernoece and their 

 allies. The simple fact that these 

 are animals of which only the 

 females ^ are parasitic is of great 

 interest ; this one-sided parasitism 

 has never yet been observed in 

 the male, except in the already 

 quoted case (p. 10, note) of Bonellia. 

 We must take into considera- 

 tion here that only in a few cases 

 is there a smaller expenditure 

 in proportion to the produc- 

 tion of sexual tissue, whUe for 

 the female, on the contrary, the 

 economic advantages of parasitism 

 are of great importance. All that 

 has been said concerning the coinci- 

 dence of sexual maturity with the 

 parasitic stage may be summed up 

 in the following sentence : — In the 

 Female, majority of parasitic animals the 

 eggs are produced, fertilised, and de- 

 posited while they are in the parasitic stage. Although it is usually the 

 case that the eggs are deposited in the host in which the parasite 

 dwells, there are a few exceptions, such as many Tcenim, where the 

 eggs remain in the proglottides and are extruded from the body of 

 their host. 



Fig. 28. 



-Pulex penetrans, 

 b. Male. 



EGGS AND EMBRYOS. 



In general the eggs of parasites are deposited in those places where 

 the parent lives ; thus the Epizoa lay their eggs upon the outer skin ; 

 the intestinal parasites deposit them in the intestine of their host, and 

 so on. In some cases, however, at the time of oviposition, parasites 

 undertake special migrations like free-living animals. There is a 

 human parasitic worm that does so — Distomum hcematobium (Fig. 

 29); this worm usually lives in the portal vein, but when sexually 

 mature, as we learn from Bilharz, migrates in pairs, the female being 



^ In the same manner the sucking of blood by the Culicidae is confined to the females. 

 The males possess a suctorial apparatus with which they can take up fluid nourishment, 

 but it is not so strongly developed as to enable them to pierce the skin. See Dimmock 

 on " The Anatomy of the Mouth-Parts of some Diptera," p. 20 : Boston, 1881. — R. L, 



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