SYSTEM OF NATURE. 77 
readily killed, a tortoise most tenacious of life. The Al¬ 
mighty has scarcely created animals whose characters 
present so complete a contrast. I cannot, therefore, avoid 
the conclusion that they must be placed at the opposite 
points of the vertebrate circle, as already suggested at 
page 70. 
The distinguishing characters of the vertebrate group 
are these : —the brain and principal trunk of the nervous 
system are enclosed in a bony envelope formed by the 
cranium and vertebrae : to the sides of this intermedial 
column are attached the ribs and bones of the limbs which 
form the frame-work of the body : the muscles generally 
cover the bones whose motion they occasion. 
