THIRD GREAT DIVISION OF THE ANIMAL 



KINGDOM. 



ANIMALIA ARTICULATA. 



This tliird general form is as well characterised as that of 

 the Vertehrata ; the skeleton is not internal as in the latter, 

 neither is it annihilated as in the Mollusca. The articulated 

 rings which encircle the body, and frequently the limbs, sup- 

 ply the place of it, and as they are usually hard, they furnish 

 to the powers of motion all requisite points of support, so that 

 here, as among the Vertebrata, we find the walk, the run, the 

 leap, natation and flight. Those families only are restricted to 

 reptation which are either deprived of feet, or in which the 

 articulations are membranous and soft. This external position 

 of the hard parts, and the internal one of the muscles, reduce 

 each articulation to the form of a sheath, and allow it but two 

 kinds of motion. When connected with the neighbouring 

 parts by a firm joint, as happens in the limbs, it is fixed there 

 by two points, and can only move by gynglymus, that is, in 

 one single plane, a disposition which requires a greater number 

 of joints to produce a same variety of motion. A greater loss 

 of muscular power is also the result, and consequently more 

 general weakness in each animal, in proportion to its size. 



But the parts which compose the body are not always arti- 

 culated in this way ; most generally they are only united by 

 flexible membranes, or they fit into each other, and then their 

 motions are more various, but have not the same force. 

 Vol. II. 3 F 



