100 THE EFFECTS OF PARASITES ON THEIR HOSTS. 



must, of course, determine many of the methods of infection. We 

 should as little think of eating a living snail as a bladder-worm. But 

 such an animal is easily hidden among salad and vegetables, which 

 we eat raw ; and this is specially likely when they have grown in 

 damp places, as is the case with water-cresses. Similarly, when eating 

 roots, fallen fruit, &c., we may easily swallow worms, insects, and other 

 small animals, without noticing it, and be infected with parasitic 

 germs of various kinds ; and where such creatures are eaten as food or 

 as tit-bits, as happens especially among savage peoples, other methods 

 of infection are obviously open. 



We know, of course, as yet but few instances which prove a 

 transmission accomplished in this way. Infection with Tcenia cucu- 

 merina must be referred to this class. This parasite occurs not un- 

 frequently in children, and is transmitted through the dog-louse 

 (Trichodectes), in which the worm passes its immature stages ; 1 but 

 this mode of transmission is probably rare and local. 



Under some circumstances, drinking water is the medium of in- 

 fection. This is especially the case when the water is derived from 

 ponds or reservoirs inhabited by numerous small organisms, or abound- 

 ing in organic remains. Small Crustacea (Cyclops) swallowed along 

 with the water transmit the Guinea-worm, which, according to Fed- 

 schenko, 2 passes its early life in these animals. And besides trans- 

 mitters of parasites, we find here and there in such water free-living 

 immature stages of such worms as Dochmius trigonocephalus, which 

 are carried with the water into the intestine, and there settle down. 3 



The older physicians were wont to emphasise the use of fruit and raw 

 vegetables as the means of introducing certain forms of worms, and this 

 is, in a certain sense, as we now know, perfectly justifiable ; but only 

 in a certain sense, for we know with absolute certainty of no instance, 

 either among men or animals, where vegetable food furnishes in itself 

 either a transmitter of parasites or a parasitic germ ; yet the possi- 



postea. ) The same may be said of the old conjecture, that the common custom of watering 

 celery beds, &c. with the liquid contents of dung-heaps resulted in the importation of the 

 eggs of BothriocepJicdus, which at once developed into tape-worms in the intestine, especi- 

 ally as the eggs of Helminths found in dung-heaps can hardly ever be in a condition capable 

 of germination. Otherwise, it might be supposed that infection with bladder-worms (not, 

 as is sometimes said, with tape-worms) was sometimes brought about in this way by the 

 eggs of Tcenia solium. 



1 See VoL I., also Melnikoff, Archivf. Naturgesck., Jahrg. xxxv., Bd. L, p. 62, 1869. 



a See Vol. II. 



3 See Vol. II. I may add that Perona and Grassi (" Sullo sviluppo dell' Anchylo- 

 stoma duodenale," Pavia, 1878) have meanwhile proved, by their researches into the 

 immature state of Dochmius duodenalis what VVucherer also (Gazeta medico, di Bahia, No. 

 65, 1869) had previously shown, viz., that the life-history of Dochmius triyonocephalus was 

 similar to that of this worm. 



