LIFE TROCESSES OF ANIMALS 361 



hold their prey with their teeth, which point backward and 

 swallow their food whole, but the birds have bills with which 

 they pick up their foot!. The mammals, as a rule, possess 

 teeth fitted either for tearing or grinding and therefore the 

 food is usually masticated. 



Digestion. — Among the most important organs in the 

 body are those that carry on the processes of digestion. The 

 number, size, structure, and appearance of the organs of 

 digestion vary greatly in the different groups of animals. 

 For example, none of the one-celled animals possess dis- 

 tinct organs of digestion. The food of these animals is 

 digested anywhere in the protoplasm. In the sponges 

 there are no distinct digestive organs, but individual cells 

 gather and assimilate their own food. The hydra has no 

 alimentary canal, but some of the cells of the endoderm 

 secrete, within the body cavity, a digestive fluid that acts 

 upon the food. The echinoderms and worms offer the first 

 examples of animals with a fully developed digestive tract, 

 or tube. The earthworm has a weU-defined alimentary 

 canal surrounsled by tlie perivisceral space, or coelorae. 

 It consists of several distinct parts and is quite similar to 

 that of the locust. The digestive organs of the different 

 members of the Mollusca vary in number and development, 

 but in the clam there is a mouth, an esophagus, a stomach, 

 and an intestine. The ahmentary canal is most highly 

 developed in the vertebrates. In the rabbit, for example, 

 it consists of three main diAdsions : esophagus, stomach, and 

 intestine. Moreover, a liver and a pancreas are present as 

 appendages of the canal, but these are organs of secretion. 



Secretion. — There are certain organs in the bo(hes of 

 the higher animals known as glands, that take special food 

 materials from the blood and elaborate them into product? 



