INSECTA, A CLASS OF ARTHROPODS 199 



amples of a nervous system and might be regarded as uncon- 

 nected nerve ganglia. 



In the worms each segment has its nerve mass or ganglion, but 

 all are connected by a double nerve fiber and each sends out many 

 branches to various organs, which are thus controlled. Then, 

 too, in the worms, there is a larger ganglion in the anterior end, 

 above the mouth, which sends special nerves to the mouth parts 

 and skin. Although there are no special organs of sensation, and 

 the structure is very primitive, there is, nevertheless, an organ 

 corresponding to a brain. 



In the Crustacea, the head ganglion, or brain, is located at the 

 base of the rostrum. It is much larger than in worms and has 

 branches extending to the eyes, ears, antennae, and mouth parts. 

 This brain is connected with ganglia along the under side of the 

 body but instead of having one for each segment, as in the worms, 

 they are combined into eleven larger and more complicated nerve 

 masses. 



In the insects this combination of ganglia has gone farther still. 

 Including the brain there are two ganglia in the head, three in the 

 thorax, and five in the abdomen, and the brain and sense organs 

 are much more specialized in function. 



If we could study more kinds of animals we would observe this 

 general tendency toward increasing the development of the head 

 ganglia, of combining others and reducing their number, while 

 increasing their ability, and the development of more efficient 

 sense organs and greater motion control. 



As soon as the simplest animal forms developed far enough to 

 have one end always go forward (anterior) in locomotion, then 

 that end, naturally, "'ran into" contact with its environment. 

 So, at the anterior end the sense organs could be most useful, which 

 is the reason for this headward tendency in development. 



In all animals the nervous system performs two general func- 

 tions; it receives and appreciates impressions from without (sen- 

 sation), and causes and controls motions from within (motor im- 

 pulses) . As the animals increase in complexity, the nervous system 

 correspondingly develops. As the complexity increased, there was 



