194 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



Reactions to Stimuli 



Earthworms live in soil and make burrows which extend from a few 

 inches to several feet under ground. They are nocturnal and lie in 

 their burrows not far from the surface during the day time, coming 

 out at night to forage for food. In winter, they go below the frost 

 line, remaining there inactive. In hot and dry weather, they go as 

 far down as possible into the earth, while a heavy rain will bring them 

 out of their burrows in great numbers. Earthworms react positively 

 to mechanical stimuli. A vibration on the earth will send them down 

 into their burrows. They are positively attracted to surfaces of solid 

 objects, as can be seen if worms are placed on moist blotting paper in 

 a covered pan. They will soon be found lying along the edges of the 

 pan, where two surfaces are in contact with the body. This response 

 to contact apparently keeps them quite constantly in their burrows. 

 They react positively to certain chemical substances, like foods, and 

 move away from others. A match that has been dipped in ammonia 

 and placed near the anterior end of an earthworm will demonstrate 

 this reaction. They respond positively to moderate moisture, which 

 is needed for respiration through the body covering, and to different 

 intensities of light, by withdrawing from bright areas and moving 

 toward weak illuminations. Like Hydra, however, reactions to 

 stimuli depend largely on the "physiological condition" of the worm, 

 that is, upon internal rather than upon external factors. 



The Nervous System and Its Functions 



The earthworm has a simple type of central nervous system con- 

 sisting of a ventral nerve cord, with thickenings, called ganglia, in 

 each segment, a dorsal "brain" or supraesophageal ganglion, made up 

 of two ganglia, and a "ring" of nervous tissue, called the circum- 

 esophageal connectives, which extends around the esophagus, connect- 

 ing the "brain" with the ventral nerve cord. Lateral nerves, which 

 leave the "brain" and cord to end in muscles, skin, and other organs, 

 form a peripheral nervous system. The worm does not have visible 

 organs of sensation, but the skin, especially at the anterior and 

 posterior ends, is dotted with groups of tiny sensory cells. Some of 

 these are sensitive to light, and still others probably to odor. Stimuli 

 received by these cells are transmitted to the central nervous system 

 by means of nerve fibers. Those which lead from the sensory cells 

 to the central nervous system are known as afferent fibers, while out- 



