248 INTRODUCTION TO GENERAL SCIENCE 



structure or are covered with some kind of shell or horny 

 substance. 



The common invertebrates are worms, insects, and the 

 crustaceans, or shellfish. See Section 178, Insects and 

 Smaller Animals. Worms are very beneficial to the soil. 

 See Section 131, Disintegration due to Plant and Animal Life. 

 References : 



1. 1304 : 359. Home of Invertebrates. 



2. 1503:191-193. Sponges. 



3. 1503:208-214. Worms. 



4. 1503:215-226. Crustaceans. 



5. 1503:256-258. Spiders. 



6. 1503:259-269. The Mollusks. 



a. 1504:304-306. The Chief Divisions of the Animal 



Kingdom. 



b. 1505 : 8-9. Method of Classifying Animals. 



c. 1505:17-21. Sponges. 



d. 1505:42-50. Worms. 



e. 1505:51-61. Crustaceans. 



/. 1508 : 1-3. Animals Classified. 



g. 1508 : 243-254. Animal Relationships. 

 h. 1512 : 284-285. Classification of Animals. 



182. ANIMAL LIFE FISHES, ANIMALS, AND BIKDS 



It is quite likely.that the first living beings of any size were 

 water animals, although not, perhaps, fishes. 



Some of these water animals went upon the land, and by 

 persistent habits, through untold ages, became adapted to 

 life on land. Others developed into birds, or at least flying 

 animals. Those species which became best adapted to any 

 particular kind of life survived and produced their kind. 

 The others died. Thus, starting as life in the water, all the 

 present forms, as well as many extinct species, have developed. 



