SOURCES OF ENERGY AND MATERIALS 



101 



rendered completely soluble and immediately ready to enter into the 

 metabolism of protoplasm. Other foods leave the digestive cavities 

 lacking still one or two of the simplifjdng steps which are necessary. 

 The cells which receive these incompletely digested foods finish the 

 process themselves. Indeed, all cells which use these kinds of foods in 

 their metabolism must have the power of taking these last digestive 

 steps. Thus some of the primitive digestive activities characteristic of 

 protozoa are not lost b}'' any active cells in any organism. 



FOOD VACUOLES 



COELENTERON 



AMOEBA 



PARAMECIUM 



HYDRA 



\PHARYNX 



GIZZARD, 



.MOUTH 



NTESTINE 



ANUS' 



BUCCAL 

 CAVITY 



.MOUTH 



EARTHWORM 



CAECUM 



^-^S' LARGE 



ccT,K. ?-~0"^ INTESTINE 



INTESTINE 

 GALL BLADDER 



ANUS, 



::p' 



RECTUM" 



SALIVARY/ 

 GLAND 



MAMMAL 



Fig. 88. — Diagrams of several types of digestive systems in metazoa, compared with 



protozoa. 



Simple Digestive Systems. — ^The simplest system in which digestion 

 occurs in a cavity is that known as a coelenteron. Hydra (Fig. 88) has 

 such a system. A coelenteron has only one opening to the outside, 

 usually called the mouth, although besides taking in food that opening 

 must also be the place of exit of undigested matter. The coelenteron 

 of Hydra is in the main a simple sac, though it is branched into the 

 ring of tentacles near the free end of the body. A less diagrammatic 

 representation of Hydra's coelenteron is given in Fig. 59, where it is 

 labeled the gastrovascular cavity and the cells forming its wall are the 

 endoderm. Flatworms also have a coelenteron. In some of them (Fig. 

 89, above) it is as simple as in Hydra, but the simplicity is not primitive; 



