b INTRODUCTION. 



each other that it is scarcely possible to find any common character- 

 istics. 



4th. The next division, that of the Radiates, is so named on ac- 

 count of the radiated or star-like form of the body, so well exhibited 

 in the Star-fishes and the Sea-anemones. 



5th. The Protozoa, or primitive animals, are, as far as we know, 

 devoid of internal organs or external limbs, and in many of them the 

 signs of life are so feeble that they can scarcely be distinguished from 

 vegetable germs. The Sponges and Infusorial Animalcules are famil- 

 iar examples of this division. 



Vertebrates. — The term " Vertebrate " is derived from the Latin 

 word vertere, signifying " to turn ;" and the various bones that are gath- 

 ered round and defend the spinal cord are named vertebrae because 

 they are capable of being moved upon each other in order to permit 

 the animal to flex its body. 



