40 ANIMAL LIFE 
speak of an oesophagus or intestine in connection with this 
most primitive of alimentary sacs. The cells which line 
the sacs show some differentiation ; some are gland cells 
and secrete digestive fluids; some are ameboid and are 
provided with pseudopods or flagella for seizing bits of 
food. The food caught by the tentacles comes into the ali- 
mentary sac through the opening or primitive mouth, and 
ee ro 
p 4) PA SAN 
| oe 
Fig. 32.—Diagrammatic sketch of a flat- Fia. 33.—Sea-cucumber (Holothurian) 
worm (Planaria), showing the dissected to show alimentary canal, 
branched alimentary canal, al. c.— al. c.—After LEvcKART. 
After Jigima and HatscHEex. 
what of it is digestible is, by the aid of the giand cells and 
the ameboid cells, taken up and assimilated, while the rest 
of it is carried out by water currents again through the 
single opening. 
In the flatworms (Fig. 32) like Planaria (small, thin, 
flattened worms to be found in the mud at the bottom of 
fresh-water ponds) the mouth opens into a short, narrow 
tube which may be called an esophagus. The wsophagus 
