ANNULOIDA : SCOLECIDA. 197 



when still extremely minute. According to Dr Bastian, how- 

 ever, 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. 72) constitute the family of the 

 Anguittulidtf) 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. 72, A) and the Tylenchus (or Vibrio] 

 tritid, 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." 



The parasitic and free Nematoids are connected together by 

 an Ascaris (A. nigrovenosd), which in succeeding generations 

 is alternately free and parasitic. This Ascaris has long been 

 known as inhabiting the lungs of the frog, but it has been 

 shown by Mecznikow that " the young of this animal become 

 real, free Nematoids ; for, after passing from the intestine of 

 the frog into damp earth or mud, they grow rapidly, and 

 actually develop in the course of a few days, whilst still in this 

 external medium, into sexually mature animals. Young, differ- 

 ing somewhat in external characters from their parents, are 

 soon produced by them, and these attain merely a certain 

 stage of development whilst in the moist earth, arriving at 

 sexual maturity only after they have become parasites, and are 

 ensconced in the lung of the frog" (Bastian). This extraor- 

 dinary history is rendered still more remarkable, if it should 

 be proved that the young of the parasitic forms of this Ascaris 

 are produced by a process of parthenogenesis \ and this seems 

 to be highly probable, since none of the individuals which are 

 found as parasites are males, but are universally females. 



