DIGESTION IN JIULTICELLULAR ANIMALS 149 



into the cavity and causes it to circle about, thus bring- 

 ing the food which it may contain in contact with all the 

 cells ■\\-hich line the cavity. Many of these cells give off 

 digestive secretions which dissolve the food matter con- 

 tained in the cavity, and thus prepare it for absorption. 

 Such cells represent simple glands, and since the solution 

 takes place outside of the cells it is called extra-cellular 

 digestion, and the cavity becomes a sort of digestive 

 cavity or tract. All animals except the protozoans have 

 extra-cellular digestion. In the forms mentioned (hydra 

 and sponge) the waste is thrown out of the lining cells 

 into the same cavity from which they absorb the food 

 and is removed by the outgoing currents of water. 



Worms. In these forms we have for the first time a 

 definite alimentary tract or canal. In some, this canal 



Fig. 51 — Diagram of a longitudinal section of an earthworm, showing the 

 central digestive tube, 



is open onl}^ at one end, while in others it forms a con- 

 tinuous tube running the length of the body with a true 

 mouth and anus. Such a continuous tube is found in 

 the earthworm. In this animal the slitlike mouth at 

 the front of the body opens into an expanded, muscular 

 walled cavity just back of it called the pharynx. B}^ 

 the action of these muscular walls the food is sucked 

 into the cavity through the mouth, after it has been 



