Uivisions of Life and Time 



an important part in the economy of nature 

 owing to the vast numbers in which they exist. 

 They are so numerous in arctic seas as to color 

 large tracts of water a pale green and to pro- 

 vide an important article of food for the great 

 Greenland whale, while their shells settle in 

 countless myriads on the sea-floor to form the 

 deposit known as pteropod ooze. 



The Arthropoda, the joint-footed animals, 

 contain besides other less familiar forms such 

 well-known groups as crabs, insects, spiders, and 

 centipeds, and comprise several hundred thou- 

 sand species, forming a prominent and impor- 

 tant portion of the living world. Doubtless all 

 these flying, creeping, and crawling things were 

 equally abundant in the past, although this is 

 not indicated by their fossil remains, since, as 

 has been said before, many were so delicate in 

 texture as to be preserved only under very 

 favorable circumstances. To this great group 

 belongs the extinct order of Trilobites, of which 

 we shall learn more later, and the great and 

 equally extinct Eurypterids. 



Going down the line, the next phylum, the 

 45 



