NERVOUS SYSTEM OF INSECTS. 549 



Talitrus, for it consists of a longitudinal series o^ 

 ganglia, usually twelve or thirteen in number, 

 connected in their whole length by a double 

 filament. By degrees the different parts of 

 which it consists approach each other, the tho- 

 racic ganglia, in particular, coalescing into 

 larger masses, and becoming less numerous, 

 some being apparently obliterated ; the whole 

 cord becomes in consequence shorter, and the 

 abdominal ganglia are carried forwards. The 

 optic nerves are greatly enlarged during the 

 latter stages of transformation, and are often 

 each of greater magnitude than the brain itself. 

 A set of nerves has also been discovered, the 

 course of which is peculiar, and appears to cor- 

 respond with the sympathetic or ganglionic sys- 

 tem of nerves in vertebrated animals, while 

 another nerve resembles in its mode of distri- 

 bution, the pneumo- gastric nerve, or par vagum. 

 Very recently Mr. Newport has distinctly traced 

 a separate nervous tract, which he conceives 

 gives origin to the motor nerves, while the 

 subjacent column sends out the nerves of sen- 

 sation. 



In the next great division of the animal king- 

 dom, 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 Asterias, where the 

 nervous system (Fig. 445) is composed of small 



