ARTICULATA. 265 



sions are well marked ; and at length, in the more 

 perfect Crustacea, such as the Crabs, almost all 

 these distinctions are lost in the broad and solid 

 shield which envelopes the whole. 



Many of the animals before us, from the supe- 

 riority of their organs, are fitted to live on the 

 land and breathe the air, but many are still aquatic 

 in their habits. We have, then, examples of all 

 kinds of motion, walking, running, leaping, flying, 

 and swimming. 



The brain is small, and sends forth two cords 

 of nervous matter, which run in a parallel line along 

 the belly, and unite at certain intervals into knots, 

 or ganglia, whence the nerves of the body and 

 limbs proceed. 



This great Division is separated into four Classes, 

 distinguished by the comparative development of 

 their structure. 



VOL. II. 



