CHAPTER III. 



THE ARTICULATA OR ARTICULATES. 

 SECTION I. 



THE ARTICULATA CONSIDERED AS A BRANCH. 



THE Branch of Articulata includes all animals which 

 are divided transversely into rings or joints more or less 

 movable upon one another, and which have no internal 



FIG. 353. 



Articulate animal. Earth-worm. Lumbr ictus lerrestris, Linnaeus. 



skeleton, but whose hard parts, even when present, are ex- 

 ternal. Hard parts are wholly wanting in many cases. 



The limbs when present are also composed of rings 

 or segments. In a word, the Articulates, in their struc- 

 ture, are essentially jointed cylinders more or less modified. 



The external covering of the Insecta and Crustacea the two highest 

 classes of the Articulates is hornlike in appearance, and consists 

 mainly of a substance which chemists call chitine. 



All articulate animals very distinctly exhibit bilateral 

 symmetry ; that is, the parts are alike on the two sides of 

 the median line of the animal. And there is also more or 

 less of antero-posterior symmetry, or correspondence be- 

 tween the anterior and posterior half of the animal. 



