DR. THEODUH NEUMANN. 219 



organs of locomotion, all the organs of sense, are gone; 

 the body is reduced to utmost implicity: we have before 

 us nothing but a bag, somewhat irregular in shape, be- 

 cause of a few appendages and unevennesses. Coincident 

 with this disappearance of movability is the complete 

 reduction of all muscles of locomotion; in short, only a 

 very small and simple remnant of the original animal is 

 left, and that has often lost all resemblance to a living 

 creature. 



These degenerative changes which occur in consequence 

 of parasitism have been described here as a single con- 

 tinuous process. They can not, of course, be observed in 

 nature, for within the short space of a human life they 

 are by no means visible; the form of the animal remains 

 constant. But if we keep in mind that of one group of 

 parasites not all began parasitic life at the same time, 

 but one earlier, another later; that perhaps one species 

 resisted those degenerative influences more than others ; 

 then we are allowed to suppose that among creatures 

 now living one kind may be more changed and degene- 

 rated than another. We are consequently able to observe 

 side by side different phases which really follow one an- 

 other, and from them we may, under favorable circum- 

 stances, put together a series which gives a very distinct 

 picture of the gradual development into a parasite. 



Thus the order of the Nematods possess still consider- 

 able mobility of their bodies and make extensive mi- 

 grations. Especially is the thread-worm (Oxyuris vermi- 

 cularis) famous on account of its nightly wanderings, 

 which it undertakes out of the intestinal canal of its 

 bearer into the surrounding exterior regions of the body — 

 wanderings which are by no means pleasant to the host, 

 since they cause a very disagreeable itching and tick- 

 line. 



In other Nematods this development of the muscular 

 system is not of so high a character, but is reduced iu 



157 



