48 



PHYSIOLOGY 



material passes out. The food is made fit for use in the 

 cavity, passes into the cells lining the cavity and thence 

 to all parts of the body. In most of these animals the 

 cavity is bag-like (Fig. 30 A), but in jellyfish it is a 

 well-defined tube (Fig. 30 B). This tube-like charac- 



Fig. 30 A.— Digestive tract of the 

 hydroid. (See Fig. 36, A.) 



Fig. 30 B.— Digestive tract of the 

 jellyfish. (See Fig. 36, A.) 



ter is a marked characteristic of the alimentary system 

 in all higher animals and gives rise to the name ali- 

 mentary canal that is often used to designate this organ 

 of the body. 



The Complete Digestive Tract. — In the next group 

 of animals a further development occurs which also be- 

 comes an important characteristic of all higher animals. 

 The tube develops a second opening to the outside. 

 Food then enters through one opening and refuse passes 

 off through the other. Special organs exist for the 

 eliminating of waste substances which result from the 

 splitting up of protoplasm. In almost all animals, then, 

 the digestive system is a tube open at both ends to the 



