ANNULOSA: NEMATODA. 



247 



mediate portion of its life in the interior of fresh-water Crustaceans (Cyclops), 

 but how it gains access to the cellular tissue of man is still unknown. Ac- 

 cording 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." 



In addition to the above-mentioned forms, it may be noted that minute 

 parasitic Nematoids are not uncommonly found, sometimes in vast numbers, 

 in the blood of various animals, including 

 the dog, man himself, and various birds. 

 Some of these haematozoa are embryonic, 

 others appear to be mature, and they may 

 or may not give rise by their presence to 

 appreciable morbid symptoms. The origin 

 of these hsematozoa, their development, and 

 the mode in which they are introduced into 

 the blood, are subjects, for the most part, 

 still shrouded in obscurity. The most re- 

 markable species is the Filaria sanguinis- 

 hominis, which in its immature state inhabits 

 the blood of man in intertropical regions, 

 its presence being commonly associated with 

 chyluria, hsematuria, and other morbid 

 affections. 



The second section of the Nematoda com- 

 prises worms which are not at any time 

 parasitic, but which are permanently free. 

 These "free Nematoids" (fig. 121) consti- 

 tute the family of the Anguillulidce, 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 feat- 

 ures of their anatomy, but they differ in often 

 possessing pigment - spots, or rudimentary 

 eyes, ir being mostly provided with a ter- 

 minal 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 free Nematoids are the 

 Vinegar Eel (Anguillula aceti, fig. 122, A) 

 and tne 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." 



The parasitic and free Nematoids are connected together by an Ascaris 

 (A. rii^rovenosa), 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, differing somewhat in external characters from 

 their parents, are soon produced by them, and these attain merely a certain 

 stage o' development whilst in the moist earth, arriving at sexual maturity 

 only after they have become parasites, and are ensconced in the lung of the 



Fig. 121. Free Nematoids. A, 

 Angtiillula ace til B, Dory- 

 laimus stagnalis. Magnified. 

 (After Bastian.) 



