NERVOUS SYSTEM OP INSECTS. 



337 



origin to the motor nerves, while the subjacent column sends 

 out the nerves of sensation. 



In the next great division of the animal kingdom, which 

 includes all molluscous animals, the nervous ganglia have a 

 circular, instead of a longitudinal arrangement. The first 

 example of this type occurs in the *ftsterias y where the ner- 

 vous system (Fig. 445) is composed of small ganglia, equal 



448 



in number to the rays of the animal, and disposed in a cir- 

 cle round the central aperture or mouth, but occupying si- 

 tuations intermediate between each of the rays. A nerve 

 is sent off from both sides of each ganglion, and passes 

 along the side of the rays, each ray receiving a pair of 

 these nerves. In the Holothuria there is a similar chain 

 of ganglia, encircling the oesophagus; and the same mode 

 of arrangement prevails in all the bivalve Mollusca, 

 except that, besides the oesophageal ganglia, others are met 

 with in different parts of the body, distributing branches to 

 the viscera, and connected with one another and with the 

 oesophageal ganglia, by filaments, so as to form with them 

 one continuous nervous system. In the Gasteropoda, which 

 are furnished with a distinct head and organs of the higher 



