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 flapping 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 
Fic. 28.—Anatomy of fresh-water flat- of which are scarcely larger 
worm (Planaria), exs, excretory sys) than many of the Protozoa, 
Cia eesti me Bnav, the alimentary canal is a sim- 
ous system, 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- 
