160 



ANARTHROPODA 



state, and no further growth occurs until the meat is 

 eaten by man in whom the larva transforms into the 

 tape form. The poisonous excretions given off by the 

 worm cause nausea, headache, and dizziness in the host. 



The larval stage of the tapeworm of the dog is found 

 in the rabbit, while the early stage of the cat's tapeworm 

 occurs in the mouse. It is therefore evident that the 

 completion of the life cycle in these parasites requires two 

 hosts. 



Fresh Water Flatworms. These are common in most 

 ponds and streams. They vary in size from minute 



specks to forms 

 more than a half 

 inch in length. 

 The whitish or 

 grayish color to- 

 gether with their 

 slow gliding move- 

 ments over the 

 surface of water 

 plants and the 

 under sides of stones and sticks are the features by which 

 they may be recognized. They move largely by means 

 of the cilia over the surface. A good plan for secur- 

 ing them is to place in a jar of water some sticks and 

 leaves taken from several quiet pools. The following day 

 they may be seen on the side of the vessel. In many 

 forms the mouth is in the middle of the under side and the 

 alimentary canal has a forward and two backward exten- 

 sions with numerous lateral branches. This group has the 

 power of regeneration to such a degree that if an individual 



FIG. 18G. Planaria ; a common aquatic flatworm. 

 The two minute dots in the head are the eyes, and 

 the black much branched structure is the alimen- 

 tary canal. Photograph enlarged four diameters. 



