318 ANIMALS 



The earthworm has a long body, divided by crosswise 

 rings. The body has a definite head (anterior) end. If 

 we were to divide the body lengthwise, from top to bot- 

 tom, the two halves would be similar. We say that such 

 a body has two-sided (bilateral) symmetry. The earth- 

 worm has a mouth at the front of the under (ventral) side 

 of the body, and a body cavity the whole length of the 

 body. Waste material is excreted through openings in 

 each side of the body "rings." There is a nervous 

 system, with a brain at the front of the body. The 

 nerve cords are on the ventral side of the body; not, as 

 in higher animals, along the dorsal side (the back). 



The earthworm has two sets of muscles in each "ring." 

 One set goes around the body. When this set contracts, 

 the ring grows smaller in diameter. The other set of 

 muscles is from front to back, in each ring. When these 

 contract, the ring grows narrower, and the whole body 

 becomes shorter. By extending a portion of the body, 

 by then holding on to the ground, and, finally, by con- 

 tracting the extended portion, the worm is able to move 

 along. 



The earthworm's blood "circulates" in two lengthwise vessels. 

 Contractions take place at certain places called ''hearts/' and form 

 waves that pass through the vessels. Respiration, or communication 

 between the blood and the air, takes place through the body wall, 

 especially at certain thin places; there are no gills or lungs. Worms 

 are reproduced by the sexual method. If a worm's head or "tail" 

 is cut off, as often happens in the plowing or spading of ground, 

 each portion can replace the rings it has lost, and produce a perfect 

 worm. 



In digging its burrows the earthworm swallows the soil, and extracts 

 whatever food the soil contains. Then it casts off the soil in little 



