ANNULOIDA: SCOLECIDA. i;i 



viviparously. The young Trichina 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 Trichina, he is now safe; since they can- 

 not become sexually mature, or develop themselves further, 

 until again transferred to the alimentary canal of some other 

 animal. 



The Guinea -worm (Dracunculus or Filar ia medinensis) is 

 a Nematode worm, which inhabits, during one stage of its 

 existence, the cellular tissue of the human body, generally 

 attacking 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 " (fig. 50) constitute the family of the 

 Anguillulida, 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-spots, or rudimentary eyes, in being mostly 

 provided with a terminal sucker, and in bringing forth com- 

 paratively 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, fig. 50, A) and the Tylenchus (or Vibrio] 



