PASSIVE MIGRATION. 



65 



A passive migration which occurs only exceptionally in 

 Entozoa with free-living embryos is the rule in those species 

 which have no free young stage. In the latter group the embryos, 

 still enclosed by the egg-shell, reach in some way or other the intestine 

 of their host ; the process of alimentation affords numerous oppor- 

 tunities for this to happen, which may recur after intervals, varying 

 according to the peculiarities of the mode of life. 



Many animals, especially smaller ones, actually use the eggs of 

 Entozoa as food. 1 have myself observed specimens of Gammarus 

 and Aselhcs aquaticus feeding upon eggs of Echinorhynchus which I 

 had placed in their aquarium ; others again take in the eggs acciden- 

 tally along with their food, in greater or less numbers, sometimes still 

 protected by the covering of the body of their parent. 



In the latter way grass-feeding ruminants are infected with the 

 eggs of several tape-worms {Tcenia serrata, T. marginata, T. ccenurus, 

 and T. echinococcus), which live in the intestine of dogs. The " pro- 

 glottides " of these worms crawl out of the fseces and deposit their 

 eggs upon grass stalks. I may also mention here Tcenia saginata 

 (mediocanellafa) of man, the eggs of which are transferred in the same 



I'lQ, 44, Proglottides of Tcenia saginata in various conditions of contraction. 



way to the stomach of the ox (Fig. 44) ; the pig generally becomes in- 

 fected with Tmnia solium by feeding directly upon human ordure, and 

 the meal-worm (Tenebrio molitor) devours, along with the excrement 

 of mice, the contained eggs of Spiroptera murina, while the larva of 

 the cockchafer takes in the eggs of Echinorhijnclius gigas with the 

 fteces of the pig. Man himself is frequently attacked by parasites in 

 the same way ; and dogs, when licking their master's hand, deposit 

 the effss of Pentastomum, which are thus easily transferred to the 

 alimentary canal. 



These few examples show how the germs of parasites are taken 

 in with food. In aquatic animals this is even more easily accom- 

 plished. In those that possess circlets of cilia or tentacles, the 

 eggs may be readily sw^^^^ ^/jj^g/^^o^^h food; and higher 



