PARASITISM 349 



and their hosts it is important to remember that the host 

 is necessary to the parasite and that his untimely death 

 may interrupt one of the developmental cycles by which 

 the permanence of the parasite is secured. The con- 

 tinuance of the parasitic relationship may therefore 

 be referred to the circumstance that the danger to the 

 host is not so great as to prevent the completion of at 

 least one generation of the parasites, and the preparation 

 of one crop of eggs or embryos for future activity. The 

 waste in parasite eggs and embryos is immense; enor- 

 mous numbers are produced, few survive. 



The assumption of parasitic existence also entails 

 structural decadence on the part of the parasites. Organs 

 of locomotion become less and less useful; organs of 

 special sense superfluous, and, taking the tape-worms 

 as examples of the extreme degree to which such decad- 

 ence may arrive, we find these animals without organs of 

 locomotion, without organs of alimentation, without 

 organs of special sense, without organs of circulation, 

 without organs of respiration, and consisting of a minute 

 head, a short neck, and a long series of proglottides or 

 segments, each of which is a kind of independent bisexual 

 reproductive animal, virtually an independent entity 

 in itself. The entire energy of the organism is thus con- 

 centrated upon the reproductive function that progeny 

 may be insured. 



REFERENCES. 



R. LEUCKART: " Die Parasiten des Menschen, " etc., Leipzig, 1881. 

 M. BRAUN: "Tierische Parasiten des Menschen," Wiirzburg, 

 1908. 



The New International Encyclopedia: Article upon "Parasites." 

 JOSEPH MCFARLAND: "Text-book on Pathology," Phila., 1910, 

 Chapter upon Parasitism. 



