INTRODUCTION. 63 



to external configuration of parts, yet even here the 

 degree of resemblance is generally analogous, both 

 as to structure and functions. 



The third general form is that of insects, worms, 

 Sfc. Their nervous system consists of two cords 

 extending along the belly, and swelled out at regu- 

 lar intervals into knots or ganglia. The first of 

 these placed on the oesophagus, though called the 

 brain is not much larger than the rest. The cover- 

 ing of their body is divided by transverse folds into 

 a certain number of rings, the teguments of which are 

 in some hard, and in others soft, but the muscles are 

 invariably attached to their interior. We often find 

 articulated limbs attached to the sides of the body 

 or trunk, but it is as frequently destitute of any. 



This division we denominate If articulated ani- 

 mals," (animalia articulata.) 



It is in these animals that we can observe the 

 transition from the circulating system in closed ves- 

 sels, to a nutritive process performed by simple im- 

 bibition, and likewise a transition corresponding to 

 this from the resparatory system in organs confined 

 to certain parts, to the same operation performed 

 through the medium of trachas, or air-vessels dis- 

 persed through the entire body. The organs of 

 taste and sight are the most distinct among the arti- 

 culated animals. A single tribe possesses those of 

 hearing. The jaws of this division, when any are 

 to be found, are invariably lateral. 



The fourth and last form comprehends the entire 

 of those animals usually known under the name of 



