70 



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 amoeboid 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 



PIG. 32. Diagrammatic sketch of a flat- 

 worm (Planaria), showing the 

 branched alimentary canal, al. c.~ 

 After JIJIMA and HATSCHEK. 



PIG. 33. Sea-cucumber (Holothurian) 

 dissected to show alimentary canal, 

 al. c. After LEUCKART. 



what of it is digestible is, by the aid of the gland cells and 

 the amoeboid cells, taken up and assimilated, while the rest 

 of it is carried out by water currents again through the 

 single opening. 



In the flat worms (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 oesophagus. The oesophagus 



