2)^The Digestive System 

 in Man and Other 

 Multicellular Animals 



THE SIMPLEST animals carry on ingestion, 

 digestion, and the other processes of nutri- 

 tion entirely within the limits of the single 

 cell. But higher animals have developed 

 specialized multicellular organs that display 

 various degrees of complexity and efficiency. 

 Moreover, these organs are subordinated 

 into definite organ systems; well-defined di- 

 gestive, circulatory, respiratory, and excre- 

 tory systems are recognizable in all higher 

 animals. First to be considered will be the 

 digestive system, which fulfills the functions 

 of ingestion, digestion, absorption, and eges- 

 tion, in multicellular animals generally. 



THE GASTRIC VACUOLES OF PRIMITIVE 

 METAZOA 



The earliest multicellular animals did not 

 develop a new type of digestive cavity. They 

 depended upon gastric vacuoles essentially 

 similar to those found among the Protozoa. 

 This is particularly true of the Porifera, a 



primitive phylum of animals commonly 

 called the sponges (Chap. 32). 



The cells of a sponge are aggregated in 

 the form of a porous tube, which may or 

 may not be complexly branched (Fig. 16-1). 

 The tube is closed at the lower end, but 

 communicates with the outside water through 



OSCULUM 



OSCULUM 

 ^ SP0NG0C0EL r^ 







ARROWS 



INDICATE THE 



FLOW OF WATER 



Fig. 16-1. The canal systems of two relatively simple 

 sponges. 



291 



