NERVOUS SYSTEM OF INSECTS. 549 



Talitrus, for it consists of a longitudinal series of 

 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 each of them 

 is often of greater magnitude than the brain 

 itself. A set of nerves has also been discovered, 

 the course of which is peculiar, and appears to 

 correspond with the sympathetic or ganglionic 

 system 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 



