THIRD 

 GREAT DIVISION 



OF THE 



ANIMAL KINGDOM. 



ANIMALIA ARTICULATA. 



THIS third general form is as well characterised as that of the Verte- 

 brata ; the skeleton is not internal as in the latter, neither is it anni 

 h i laird as in the Mollusca. The articulated rings which encircle the 

 body, and frequently the limbs, supply 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 articu- 

 lations arc membranous and soft. This external position of the hard 

 parts, and the internal one of the muscles, reduce each articulation to 

 tin- 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 

 tin- limbs, it is fixed there by two points, and can only move by gyn- 

 dvmus, that is, in one single plane, a disposition which requires a 

 i t<T 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 articulated in 

 this way ; most generally they are only united by flexible membranes, 

 or tli \ tit into each other, and then their motions are more various, 

 Imt have not the same force. 



The system of organs in which the Articulata resemble each other 

 tli.- most, is that of the nerves. 



Th. MI brain, which is placed on the esophagus, and furnishes nerves 

 to the parts adhriinp to tln> hrad, i.-. very small. Two cords which 

 embrace thr esophagus are extended along the abdomen, and united 



