ARTICULATA. 21? 



Order 4. Acanthocephala ; (akantha, a thorn; cephale, 

 head,) are rounded, parasitic worms, having a protrusible 

 proboscis, armed with recurved hooks. Their structure 

 is not unfrequently as simple as the Protozoa, having no 

 alimentary canal whatever, and subsisting by abrorption. 

 Like the Tape-worms, they develop through an alterna- 

 tion of generations. 



Order 5. Gordiacece. The horse-hair-like worm found 

 in rain pools is an example of this order. It begins life 

 as a little larva in mud or water pools. By means of its 

 boring spines it pierces the body of a grasshopper, bee- 

 tle, or other insect, where it becomes encysted; and 

 grows often ten times as long as its host, when it be- 

 comes free and aquatic, and produces its eggs. Some of 

 these, as the Mermis albicans, multioly so rapidly as to 

 give rise to a popular belief that they fall as " worm- 

 rains." They have remarkable tenacity of life, as they 

 can be dried into brittle threads, and yet become active 

 on being moistened. 



Order 6. Nematoidea, (nema, thread ; eidos, form.) 

 Thread-worms, or round worms. These are both free 

 and parasitic. Some of them, as the Ascaris lumbticoi- 

 des y or common round worm, often infests the small 

 intestines of children, while the Trichina spiralis, a mi- 

 nute worm found encysted in the flesh of swine , when 

 introduced into the human body, multiplies so rapidly 

 in the muscles as to give rise to dangerous, and even 

 fatal symptoms. The " eels " in vinegar and sour paste 

 also belong to this order. 



Order 7. Rotifer a, or Wheel Animalcules. These are 



microscopic in size, but so transparent that the details 

 19 



