MULTICELLULAR ANIMALS 



155 



otherwise fragmenting the food. In the Annelida, as in the earth- 

 worm (Fig. loi) the incoming food is diluted and softened by 

 secretions from glands located near the mouth whose special func- 

 tion it is to manufacture certain digestive enzymes. It is broken up 

 by abrasion with sand particles in a special region of the digestive 

 tube called the gizzard. In the Arthropoda, particularly in such 

 forms as the lobster (Fig. 102), hard, chitinous jaws grind the food, 



OeSOPHACUS GASTRIC MILL 



STOMA CH 



INTESTINE 



CAECUM 



ANUS 



Fig. 102. — Digestive system of a lobster. Note the gastric mill, consisting of rough 



plates of chitin. 



which is then subjected to a second abrading by file-like plates in 

 the stomach (the gastric mill). In the vertebrates in many cases the 

 mouth is equipped with modified scales, called teeth, that act in 

 various ways in grasping and grinding foods. 



The Human Tooth. The human tooth (Fig. 103) is homol- 

 ogous with the scale (Fig. 91) in the exoskeleton of the primitive 

 fishes, for example, the shark. It consists of two portions, the 

 ENAMEL, which is formed during development by the metabolism of 

 the ectodermal cells, and the dentine, which is comparable to the 

 bony plate of the shark scale and is formed by mesoderm. The 



