THREAD- WORMS. 



If they fall in with one, they start to creep up it, and mounting ever higher and 

 higher, as the corn grows, ultimately succeed in reaching the summit. They then 

 attack the soft 

 grain, bore into 

 it, and form gall- 

 like swellings, in 

 the middle of each 

 of which there is 

 a larval worm. 

 Here the worms 

 quickly develop to 

 normal perfection, 

 and after the 

 females have laid 

 a large quantity of 

 eggs, both they and 



the males die. VINEGAR-EEL (much enlarged). 



Subsequently the 



eggs hatch, and the larvae, which constitute the powdery substance Deferred to 



above, make their appearance. Somewhat similar diseases are produced in other 



grains by members of the same family ; and the turnip-eel (Heterodera) is very 



destructive to root-crops. 



Of the parasitic forms, the genus Rhabdonema has a remarkable course of 

 development, one species (R. nigrovenosum), which is about three-quarters of an 



a, FEMALE OF Rhafoditis FORM OF Rhabdonema nigrovenosum ; b, BROOD-POUCH. (Enlarged.) 



inch in length, living, sometimes in great numbers, in the lungs of frogs. This 

 species is hermaphroditic, and produces innumerable young ones, w^hich bore their 

 way from the lungs into the alimentary canal of their host, whence they are 

 expelled with the remains of their food. They then develop in a few days into 

 free-living, separately-sexed individuals, bearing a close resemblance to another 

 free-living worm (Rhabditis). These individuals breed; the females bear one 

 or two young apiece, and these, after devouring their mother's vitals, and making 

 their escape by bursting through the skin of her body, pass through a frog's mouth 

 into its lungs, and become the hermaphrodite adult. Another species (R. strongy- 

 loides) is of interest, inasmuch as it is parasitic in man in warm climates. 



Two more remarkable Nematoids may be mentioned, both of which infest 

 insects. The first of these, Atractonema gibbosum, is found in numbers in the 

 body -cavity of the larval and adult stages of the midge ; the completely-formed 



