200 



ASSOCIATION OF ORGANISMS 



flukes may be either internal or external parasites; the 

 tapeworms invariably inhabit the digestive tract. Many 

 species of thread worms (Fig. 51) are likewise parasitic, 



Fig. 49. — Doddor parasitic on cockle-burr. 



and while a few infest plants the greater number live in 

 animals and are in se\Tral cases responsible for certain 

 human diseases. Lice, fleas, and ticks are also dreaded 

 pests in many instances, especially owing to the fact 

 that they carry disease-producing organisms. 



