CHAPTER III. 



1HE BRANCH OF ARTICULATA, OR ARTICULATES. 



THE Branch of Articulata comprises all animals which 

 possess bilateral symmetry, and which are divided trans- 

 versely into rings or joints more or less movable upon 

 each other, and whose hard parts are external. The ali- 

 mentary canal lies in the centre of the body, and above 

 it the dorsal vessel or heart. The nervous system con- 

 sists of a sort of brain, which lies above the oesophagus, 

 from which two threads, passing around the oesophagus, 

 extend beneath the alimentary canal, along the floor of 

 the general cavity of the body, and connect at certain 

 distances small nervous centres or ganglia, whence arise 

 the nerves of the body and limbs. Each of these nervous 

 centres seems to fulfil the functions of a brain to the sur- 

 rounding parts, and preserves their sensibility for a greater 

 or less length of time after the animal has been divided. 

 The number of these nervous centres generally corre- 

 sponds to the number of the segments of the body. Ar- 

 ticulates are divided into three classes, Insects, Crus- 

 taceans, and Worms. 



SECTION I. 



THE CLASS OF INSECTS. 



THE Class of Insects comprises articulates whose res- 

 piratory apparatus consists of air-holes, called stigmata, 

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