ANNULOIDA: SCOLECIDA. 153 



viviparously. . The young Trichinae thus produced perforate 

 the walls of the alimentary canal, and, after working their way 

 amongst the muscles, become encysted. If the animal in which 

 these changes go on has sufficient vitality to bear up under 

 the severe symptoms which are produced by the migration of 

 the Trichinae,, he is now safe ; since they can not become sex- 

 ually mature, or develop themselves farther, until again trans- 

 ferred to the alimentary canal of some other animal. 



The Guinea- worm (Dracunculus or Filaria medinensis) is a 

 Nematode worm, which inhabits, during one stage of its ex- 

 istence, the cellular tissue of the human body, generally attack- 

 ing the legs, and often attaining a length of several feet. All 

 known specimens of this parasite are impregnated females, 

 containing a large number of young. The worm remains im- 

 bedded in the body, in a more or less quiescent condition, for 

 a year or more, at the end of which time it seeks the surface, 

 in order to get rid of its young. No external aperture to the 

 genital organs has hitherto been proved to exist, and it seems 

 possible that the young are produced within the body of the 

 parent by a process of internal gemmation. The young Filaria 

 consists of a vermiform body, terminating in a hair-like tail, 

 and when set free from the parent, its further development 

 probably takes place in water, when it is believed to be con- 

 verted into one of the ' Tank- worms,' so common in India. In 

 this condition it is possible, as some believe, that sexual organs ' 

 are developed, and that the females are impregnated. The 

 worm is believed to gain access to the body of bathers, when 

 still extremely minute. According to Dr. Bastian, however, it 

 appears probable that the Guinea- worm 'is a parasite only 

 accidentally, and that it and its parents were originally free 

 Nematoids.' 



The second section of the Nematoda comprises worms, which 

 are not at any time parasitic, but which are permanently free. 

 These ' free Nematoids ' constitute the family of the Anguillu- 

 lidce, of which about two hundred species have been already 

 described, mostly inhabiting fresh water or the shores of the 

 sea. They resemble the parasitic Nematoids in all the essential 

 features of their anatomy, but they differ in often possessing 

 pigment-kpots, or rudimentary eyes, in being mostly provided 

 with a terminal sucker, and in bringing forth comparatively 

 few ova at a time ; the dangers to which the young are exposed 

 being much less than in the parasitic forms. Amongst the 

 more familiar Nematoids are the Vinegar Eel (Anguillula aceti) 

 and the Tylenchus (or Vibrio) tritici, which produces a sort of 

 excrescence or gall upon the ear of wheat, causing the disease 

 known to farmers as the ' Purples,' or ' Ear Cockle.' 



