A PLANARIAN (Flat Worm). 



Phylum IV, PLATYHELMINTHES; class i, TURBELLARIA; order 2, 



TRICLADIDA. 



HABITAT. These small flat worms occur very generally in fresh- 

 water ponds. They may be found on the mud of the bottom, on 

 water plants, or on the under side of stones. Some genera are marine, 

 certain species living as external parasites on the book gills of the King- 

 crab. 



APPEARANCE. Fresh- water Planarians are very thin, flat little 

 animals one-half an inch or less in length. They are comparatively 

 broad, especially at one end. In color, shades of brown, reddish, or 

 gray predominate. They move about sometimes with a steady gliding 

 motion, at other times with more of an apparent muscular contraction. 



Technical Note. Place specimens of Planaria in a watch glass, containing 

 a little water, for study with a hand lens or the dissecting microscope. 

 A good view of the ventral surface can be obtained by placing one of the 

 animals in a drop or so of water on a slide and then turning the latter 

 over. When smaller detail is to be studied mount a specimen and 

 examine with higher power of the microscope. Clearer definition can 

 sometimes be secured by using reflected instead of transmitted light 

 from the mirror. 



GENERAL STRUCTURE. Is the symmetry of the animal bilateral 

 or radial ? Could you cut it by a plane into two similar halves ? Note 

 the movements before deciding which is the anterior and which the 

 posterior end. Usually the former is the broader and more rounded. 

 The surface on which the worm is creeping is, of course, the ventral; 

 the other is the dorsal surface. 



Near the anterior end, on the dorsal surface, are two dark-colored 

 eye-spots. Search for the mouth in the middle of the ventral surface. 

 It opens into a pharynx which is protrusible, forming, when thrust out, 



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