A STUDY OF THE NERVOUS SYSTEM 287 



9. A COMPARATIVE STUDY OF THE NERVOUS SYSTEM 



Nerve Functions in Amoeba. It is, of course, impossible 

 to speak of anything like a nervous system in single-celled 

 animals. We found, however, that the amoeba could move 

 without any appendages, could take in food without any 

 mouth, and could carry on the processes of digestion, respira- 

 tion, and excretion without any stomach, lungs, or kidneys. 

 So, too, this bit of protoplasm, although it is without a 

 nervous system, has what we may at least call nervous 

 irritability. If the slide over which an amoeba is crawling 

 be suddenly jarred, the animal will pull in its false feet 

 and assume a spherical form. It can distinguish particles 

 of food from bits of sand, for it will surround the former 

 with its body protoplasm ; dirt particles, on the other hand, 

 are passed by. The amoeba is also affected by different 

 colors of light. It moves about most vigorously when yellow 

 ra} r s are thrown upon it ; in the presence of violet light it 

 remains quiet. All these facts prove conclusively that 

 the amoeba possesses something at least akin to nervous 

 functions. 



The Nervous System of the Earthworm The " brain " of 



the earthworm consists of two small pear-shaped ganglia 

 placed end to end across the dorsal surface of the esophagus 

 (see Fig. 38). Several nerves connect this part of the nerv- 

 ous system with the sense organs on the anterior end of the 

 body. From each of the brain ganglia we have mentioned, a 

 nerve trunk runs around to the ventral surface of the esoph- 

 agus. There the two meet and run as a double chain to 

 the posterior end of the body. In each segment this nerve 

 chain has an enlargement or ganglion (each of which really 

 consists of two parts). Nerves are given off in pairs along 

 the side of this chain of ganglia, and connect this central 

 nervous system with the sense organs on the surface of the 

 body and with the muscles. 



Nervous Functions in the Earthworm. Earthworms rarely 



