82 



AKTICULATED ANIMALS. 



nervous system, wherever it has been at all dis- 

 cernible, has existed only in the form of slender 

 threads, without being accumulated into masses, or 

 centres of perception. In all creatures, however, 

 belonging to the articulate division of the animal 

 kingdom the nervous system is arranged upon a plan 

 which is sufficiently conspicuous throughout the entire 

 series. A double chain of brains, or ganglia, runs 

 down the central line of the body beneath the 

 alimentary canal ; and it is from the symmetry con- 

 spicuous in the arrangement of these that the most 

 unmistakable character whereby the articulata are 

 distinguished is furnished. 



The first pair of brains or ganglia is always situated 

 in the head, and supplies nerves to the eyes, to the 

 antennae, and to all the principal instruments of 



sensation ; and on the pro- 

 portionate size and develop- 

 ment of these ganglia the 

 perfection of the senses pos- 

 sessed by any of these crea- 

 tures depends, consequently 

 they are generally spoken 

 of as the brain. 



All the other ganglia are 

 arranged in a double series 

 along the floors of the dif- 

 ferent segments of the body, 

 each supplying the muscles 

 belonging to the rings in its 

 neighbourhood. In propor- 

 tion to the size and per- 

 fection of these ganglia, 

 therefore, will be the en- 

 ergy of the creature's 

 movements. In the an- 

 nexed engraving (Fig. 57), 

 representing the nervous 

 system of a leech and of a 



FlG. 57. NERVES OF LEECH AND 

 COCKCHAFER. 



