228 ENTO-PARASITES AND HYGIENE. 



This advantage of having the eggs of an individual 

 fructified by the spertna of another is impossible in 

 worms which are thoroughly sessile ; because they can- 

 not move, they are only able to fructify their own eggs. 

 Yet that was not sufiicient for a length of time; they 

 needed "fresh blood," a "renewal of blood," and soon 

 special arrangements were made for the copulation with 

 other individuals. This is done by free-living larvse. 

 Such are always to be found when the sessile animals 

 developed from the former, and they are essentially 

 necessary, because only in this way an extension of the 

 species may be possible. All sessile parasites have a free- 

 living early stage, which settles in a proper place after 

 some time, and then reaches its full development and 

 sexual maturity. Part of these free-living early stages, 

 however, obtained such sexual maturity even earlier still, 

 during their free life, and they were now able to transfer 

 their germinal matter upon a sessile companion. If we 

 ask which of the two sexes was prominently suited 

 for such a part, we shall learn that the male animal had 

 to take that task upon itself, since among nearly all 

 lower forms of animal life the females, as bearers of many 

 and heavy eggs, possess a much more awkward and 

 clumsy shape and less movability. Moreover, the ma- 

 turing of the sperma takes place always somewhat earlier 

 than that of the eggs. Even in immature hermaphro- 

 ditic forms the important question must have been how 

 to deposit the male sexual products of the free-living in- 

 dividuals on the female organs of the sessile animals. 



In developing this state of things further, the female 

 element could be suppressed completely, and there re- 

 mained at last only freely moving male individuals 

 which served as sperma-depositors upon the females and 

 then had fulfilled all that was required of them. Here 

 we have within the same species two entirely different 

 forms which come from equal larvae : the principal form 



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