TRICHINA, HOOKWORMS, FILARIA, ETC. 87 



and do not injure the vegetables. No harm will come from 

 eating vegetables that have been visited by these parasites. 



The Acanthocephala (Gr. akantha, thorn; kephale, head), or 

 thorn-headed worms, include a number of parasitic forms 

 which show extreme specialization in their mode of life. The 

 anterior end is developed into a conspicuous spiny organ for 

 holding on to the walls of the alimentary canal of the hosts in 

 which they live. There is no mouth or digestive organs but 

 the parasite takes its nourishment through 

 the body-wall from the food surrounding it. 



Echinorhynchus gigas, infesting hogs, is 

 the best known species of this class. In 

 America the larvae of the June beetle, Lack- 

 nosterna, which is the common white grub 

 found in the sod in pasture lands and else- 

 where, serves as the intermediate host for 

 the parasite. Hogs should not be allowed 

 to pasture on lands where the grubs have be- 

 come infected from previously infested hogs. 

 Once pasture land has become infected it 

 should be left for three years to insure the 

 maturing of all the grubs that are infected. 



Several other species belonging to this 

 class are parasites of fishes. living specimen.) 



ANIMALS OF UNCERTAIN RELATIONSHIP 



There are two groups of aquatic animals the exact relation- 

 ships of which are by no means agreed upon by systematic 

 zoologists. They are usually supposed to be more nearly 

 related to the worms than to any other group, and as they are 

 not of enough economic importance to be given a separate 

 chapter they may be mentioned here. 



The Wheel Animalcules, or Rotifers, branch Trochel- 

 minthes (Gr. trochos, wheel; helmins, worm). These are 

 minute aquatic animals which on account of their size were for 

 a long time classed with the Protozoa. But they are really 

 very complex in structure. The anterior end is provided with 



FIG. 30. A 

 wheel animalcule, 



