50 THE POPULAR SCIEXCE MOXTHLY. 



The general plan of the nervous organization throughout this lavo-c 

 division is that just described, and commencing with cephalous ani- 

 mals. It consists of symmetrically-arranged ganglia a sensorium, 

 and the necessary communicating nerves. 



And what are the actions by which this improved nervous system, 

 characterizing all this large class of animals, manifests itself? 



Taking the two extremes in the class of well-characterized articu- 

 lates, the centipede at the lower, and wasps and bees at the upper, we 

 notice a wide difference in the complexity of the general oro-anization 

 and a difference equally marked in the character of the corresponding 

 actions. In the centipede, the general structure consists of a head 

 furnished with certain organs of special sense, and a body made up of 

 a series of segments each, with the exception of the last, being a 

 repetition of the others, and each being furnished with a single pair 

 of legs. 



Fig. 2. SEJisoRrcTM and Connected Gakglia of Centipede. 



1^ 



To preside over these organs of locomotion, the nervous system 

 is distributed as follows : Each segment is suj^plied with a double 

 ganglion, or nerve-centre, all being arranged in regular order just be- 

 neath the alimentary canal, and in the head is placed the sensorium ; 

 all the different ganglia are connected by a fine double-nerve filament 

 called the ventral cord, and through this means also all are in com- 

 munication wath the sensorium. So each ganglion is in commimication 

 ^\\\h its own particular pair of locomotive organs, with each of the 

 other ganglia, and with the sensorium. 



The chief action of this animal consists in the movement of these 

 organs of locomotion ; and this it does in response to direct irritation 

 from without, or in consequence oi ' sensations received at the sen- 

 sorium through the organs of special sense, and thence transmitted 

 to the several ganglia. So long, then, as this nervous communication 

 remains unbroken, the action of all the segments composing the body 

 is harmonious, and is performed with a direct relation to the guiding 

 influence of the sensorium. This is the normal action of the animal. 

 But suppose, now, that the head, with the sensorium, be removed while 

 the creature is in motion, the legs still perform their oflice, car- 

 rying the body forward in its accustomed manner until it meets an 

 obstruction, when its progress is stopped, though the legs still con- 

 tinue to move. If the body be divided, similar results follow : the 

 forward part moves on under the guidance of the sensorium, avoiding 

 or overcoming obstacles according as sight or other sensations influ- 



