16 THE ORIGIN OF MAN 



They were sexed animals, and their bodies were 

 made up of segments, many of which articulated 

 upon one another. These segments were divided 

 into three divisions. The under, or ventral, side 

 with the limbs had a very thin outer shell, while 

 the carapace or upper shell was thin, and made 

 up of chitin, a nitrogenous substance which great- 

 ly resists chemical alteration and for this 

 reason trilobites are so often preserved as fossils, 

 Trilobites inhabited only marine and mainly in 

 shallow waters. Most of them were good swim- 

 mers, some crawled over the bottom, while others 

 lived partly buried in the muds. Many or most 

 of the trilobites could roll up their bodies for the 

 protection of the softer and more delicate parts 

 of the ventral side, thus presenting to the enemy 

 the hard thick carapace. 



Shelled animals as a rule are generally divided 

 into two classes, the Brachiopoda and the Mol- 

 lusca. 



The word Brachiopoda means arm-footed and 



