AN EARTHWORM 245 



of a nerve cell, the nucleus of which is in the central nervous 

 system. Nerve cells of this type are called motor nerve cells. 

 Mingled with them is another type of nerve cell, which has 

 its nucleus in a sense organ at or near the skin of the animal 

 and with the terminal portion of its principal fiber extend- 

 ing into the central nervous system. Cells of this type are 

 called sensory nerve cells. 



Reflex Action. In case an object touches one of the fine 

 sensory hairs on the skin of an earthworm, a message, or 

 impulse, is sent along the sensory fiber to the ventral nerve 

 chain. There the impulse is transferred to the motor nerve 

 cells. When the impulse reaches the end of the motor fiber, 

 the muscle fiber to which it is attached contracts. When 

 many sensory nerve cells are stimulated in this way, many 

 motor cells will carry out the transferred impulse, the 

 muscles will contract, and the animal moves away. This 

 kind of nerve action is called reflex action because, in a 

 sense, the impulse is reflected back from the ventral 

 nerve chain. 



Reproduction. Every earthworm contains both sper- 

 maries and ovaries. As explained on page 206, animals 

 which have the male and female glands in the same body 

 are called hermaphrodites. There are two pairs of sper- 

 maries in an earthworm, hidden in the three pairs of seminal 

 vesicles, indicated in Fig. 128. The single pair of ovaries is 

 very small. Although each earthworm produces sperma- 

 tozoa, or sperm, and eggs, the eggs of one individual are 

 always fertilized by the sperm of another. 



Reproduction takes place chiefly in the spring. On warm 

 spring evenings the worms crawl from their burrows and 

 lie stretched out on the surface of the ground with the tail 

 still holding tight in the mouth of the burrow. While in 

 this position two individuals may become paired in the act 

 of copulation. The two worms have the ventral surfaces to- 



