THREAD-WORMS. 679 



alimentary canal traversing the body nearly from end to end, and there is 

 an excretory system, consisting of tubes lying lengthwise, which com- 

 municate with each other in the fore part of the body and open upon its 

 lower surface by a single pore. The sexes are generally distinct. 



Although a large majority of the species of this class are parasitic, spending 

 the whole or part of their lives within the bodies of other animals, a few, 

 nevertheless, are to be found on plants, on fresh water or damp earth, or even 

 in the sea. But amongst the parasitic forms we meet with species that are 

 free during a part of their existence ; that is to say, either during the 

 immature stages of life or after maturity is attained . Such species as these 

 thus show an interesting gradation in habit between those kinds that are 

 wholly independent and those that live permanently in other organisms. 



One of the best known of the free living species is the Vinegar or Paste- 

 eel (Anguillula aceti), which feeds upon the minute fungi found growing in 

 these substances. Allied forms, known as Leptodera and Pelodera, occur in 

 water or damp earth ; but, unless supplied with plenty of nitrogenous food 

 in the form of decaying animal or vegetable matter, they remain in the 

 larval stage, a supply of food of this nature being necessary for the attainment 

 of maturity. 



Before passing to the most interesting members of this class, namely, those 

 that are parasitic in animals, and especially in mankind, brief reference may 

 be made to a couple of species which infest plants and are of considerable 

 economic importance on account of the damage done by their agency to corn 

 crops and vegetables of the turnip kind. Adult examples of the Wheat-eel 

 (Tylenchus tritici) attack developing ears of wheat, converting them into a 

 kind of gall. Here the young are hatched, ultimately reaching the ground 

 by the falling of the grain. They are then set free in the moist earth, and 

 wander hither and thither in search of the young shoots of growing corn. If 

 successful in their quest they ascend the stalks, and, finally reaching the 

 summit, take up their abode in the blossom of the wheat ; and, nourishing 

 themselves upon it, arrive at maturity and then produce new generations. 

 The second vegetable pest to be noticed is the Turnip Thread-worm 

 (Heterodera schachti), which gives rise to swellings on the roots of the sweet 

 turnip or sugar-beet. These swellings are caused by the expansion of the 

 female worm into a spherical body owing to the quantity of nourishment 

 she absorbs. Here the eggs are laid, and after being retained for a time in 

 a special brood-pouch, pass to the outer world, and undergo the first stages 

 of their development. Subsequently the females bore their way into the 

 turnip roots by means of a special instrument with which the throat is fitted 

 fur the purpose. 



Passing now to the species that live parasitically in other animals, we find 

 the simplest life-history in such kinds as Ascaris lumbricoides and Oxyuris 

 vermicularis, known to medical men respectively as the " round- worm '' and 

 the ** thread- worm," and occurring not infrequently in the human intestine, 

 especially during childhood. The eggs of these worms are discharged from 

 the alimentary canal of their host, and make their way back again through 

 the mouth in connection with food or impure water. In addition to man- 

 kind, species of thread- worms allied to those just mentioned infest the 

 intestines of horses, pigs, dogs, cats, and a variety of other animals. 



In addition, however, to the alimentary canal, other organs of man are 

 liable to the attacks of various Thread- worms. The Guinea or Medina-worm 

 (Dracunculus medinensis), for example, reaches maturity in the subcutaneous 



