MISCELLANEOUS ANIMALS 429 



dark in color and continually writhing and twisting about, 

 tying themselves into knots. This peculiarity has given 

 them the generic name " Gordius," from the Gordian knot. 



In a word, the life story of one of our common forms is 

 simply this : The mature worm lives in the ground and 

 comes out into the pools to lay its eggs. The egg is very 

 minute, and the tiny worm that hatches from it bores its 

 way into some insect, usually a grasshopper, and lives as 

 a parasite within its body. The insect dies, and the worm 

 enters the ground to pass the winter. 



The nematodes, to which the hair worms belong, are a 

 large class of lowly organized worms. Most of them live 

 in water, soil, or decaying matter and are harmless. 

 Many are almost or quite microscopic in size, the " vinegar 

 eel " being one of our most common forms. A few are 

 parasitic in animals, living either in the intestine or in 

 the flesh. It is these latter, especially the trichina of 

 pork, that make it unsafe to eat meat that has not been 

 thoroughly cooked. Other nematodes are parasitic in 

 plants, especially in the roots, where they produce swell- 

 ings or galls. They are particularly destructive in the 

 greenhouse and window garden in the North, where the 

 eggs are killed by freezing during the winter, but they 

 often seriously injure field and garden crops farther 

 south.i 



Mollusks. — These form one of the largest animal groups, 

 there being 21,320 living species and an almost equal 

 number (20,895) of extinct fossil species. How many 

 kinds do the children know-.' They may be defined as 



^ George E. Stone and Ralph E. Smith. " Nematode Worms,'' Bulletin 

 No. sSi Massachusetts Agricultural College, Amherst, 1898. 



