502 THE ANIMAL PARASITES OF MAN 



intestine, but in the organs of the human body that are beyond the reach of 

 external contact, parasites are found whose origin baffles conception, and most 

 remarkable suppositions and opinions have arisen regarding this origin. The 

 most simple and most natural explanation of this secret of nature was the 

 assumption of a generatio cequivoca. It was believed that spontaneous gen- 

 eration, which was supposed to be common among the lower animals, occurred 

 also among the entoparasites. Endeavors were made to demonstrate that the 

 parasites developed from the intestinal mucus, from inspissated fecal masses, 

 and from decomposed blood. Step by step, the proof was slowly furnished 

 that not only highly organized animals, but also the lower were endowed with 

 sexual powers, and that the parasites developed only from fecundated ova. In- 

 vestigators have had to contend with one great difficulty, namely, the occur- 

 rence of young, asexual forms. But the transition of these into intermediate 

 hosts, their return to the specific host, and their development into mature forms 

 was finally established experimentally. 



With certain exceptions, sexual maturity and parasitism coincide for those 

 entoparasites which are under discussion. The ova or embryos of the mature 

 parasites reach the outside world, and here, protected by a covering which is 

 very resistant to external influences, they continue their development, and 

 finally reach an intermediate host. Many exceptions occur. The embryos of 

 the trichina spiralis pass from the intestinal wall of the host immediately into 

 his tissues. Others, slipping from the sac of the ova, live free for some space 

 of time; still others migrate directly into the final host, there to reach their 

 maturity. Asexual forms first pass into an intermediate host, where they are 

 harbored and still further developed, until at last they reach their final host; 

 or, after the lapse of some time, they become subject to retrogressive changes. 



Our knowledge of the distribution of the animal parasites in man is very 

 incomplete. It is certain that oxyuris, ascaris and trichocephalus, as well as 

 the ectoparasites, pediculus, cimex and pulex, are found throughout the world, 

 and are probably the most common animal parasites. Others, such as 

 echinococcus, ankylostoma, filaria, are observed only in certain countries, out- 

 side of these only sporadically, and then introduced from infected districts. 

 Precise statistical reports regarding the distribution of the parasites, which 

 might assist in the explanation of various questions, are very limited in 

 number. 



Aside from the influence of climate and soil, the frequency of the occurrence 

 of parasites in man is due also to the customs and habits of a country, and, 

 no less, to the personal cleanliness of the individual inhabitants. I do not 

 refer merely to ordinary cleanliness of the body, which is of importance in the 

 transmission of the ectoparasite, but particularly to cleanliness in kitchen and 

 household. There can be no doubt that the majority of the parasites are trans- 

 mitted by food and drink, by ingestion from contaminated vessels, soiled 

 hands,- etc. In countries where, as in Abyssinia, beef is eaten raw or at least 

 not^ sufficiently well cooked, the beef tape-worm is greatly disseminated. The 

 tenia solium is not rarely found in Central Germany, where raw or half-cooked 

 pork is often eaten. For the same reason endemics and epidemics of trichinosis 

 have not yet been completely exterminated in these districts. The intake of the 



