46 



ANIMAL FORMS 



season, and at such periods they may be found moving about 

 with a steady gliding motion (due to cilia covering the en- 

 tire body), varied occasionally by a looping, caterpillar move- 

 ment, or by swimming with a napping of the sides of the 

 body. When watched at such times they may sometimes 

 be seen to snatch up small worms, snails, small crabs and 

 insects, which serve as food. 



More closely examining one of these forms, for example, 

 the species usually found on the under side of sticks and 

 stones in our shallow fresh-water streams (Fig. 27, A), we note 

 that the forward end is not developed into a well-defined 

 head as in the higher worms, 

 but is readily determined by 

 the presence of very simple 

 eyes and tentacles, while the 

 lower creeping surface is dis- 

 tinguished by a lighter color 

 and the presence of the 

 mouth. Through this small 

 opening a slender proboscis 

 (in reality the pharynx) may 

 be extended some distance, 

 and may be seen to hold the 

 small organisms upon which 

 it lives until they are suffi- 

 ciently digested to be taken 

 into the body. 



44, Digestive system. In 

 the smaller flatworms, some 

 of which are scarcely larger 

 than many of the Protozoa, 

 the alimentary canal is a sim- 

 ple unbranched tube ; but in 

 the larger forms such an ap- 

 paratus is replaced by a greatly branched digestive tract 

 which furnishes an extensive surface for the rapid absorp- 



FIG. 28. Anatomy of fresh-water flat- 

 worm (Planaria). exs, excretory sys- 

 tem, with flame-cell (/). The ali- 

 mentary canal is stippled. B, nerv- 

 ous system. 



