214 Education through Nature 



salts, lime, etc. The organic foods may be either 

 of animal or of vegetable origin. 



Many insects, as the beetle, butterfly, and most 

 mammals, are herbivorous. Many protozoa, most 

 coelenterates, echinoderms, mollusks, Crustacea, some 

 insects, arachnids, fishes, amphibians, reptiles, some 

 birds, and the clawed mammals, are carnivorous. A 

 few animals, including man, are omnivorous. 



The food of animals is a matter for observation 

 and study, since it not only influences the character 

 of the animals as regards its utility to man, but is sug- 

 gestive in connection with special adaptations for 

 obtaining food and for preparing the same for nutri- 

 tive uses in the body. 



The alimentary canal is the organ into which food 

 is taken. It exists in all animals except protozoa, 

 sponges, and some parasitic forms like the tapeworm. 

 In these food is absorbed through the outer layers of 

 the body. In all other metazoa the alimentary canal 

 is either a simple bag with a mouth-opening or else a 

 tube passing either straight through the body or 

 variously coiled and enlarged into pouches, such as 

 the stomach. The simpler the animal, usually, the 

 simpler the alimentary canal. Thus in hydra and 

 corals it is a mere bag, not distinctly separated from 

 the body-wall. Herbivorous animals usually have a 

 more complex alimentary canal than carnivorous, 

 the stomach in the latter being less developed, and 

 the intestines shorter. 



The length of the alimentary canal, therefore, 

 varies with the diet, and also with the habits of the 

 animals as regards its mode of obtaining its food, 

 whether by its own efforts or through the means of a 

 host, as in the case of parasites. Much can be inferred 

 in regard to the internal structure of animals from an 

 examination of their teeth, there being usually remark- 



