162 BIOLOGY FOR BEGINNERS 



crop, stomach, and long, straight intestine, terminating at the vent. 



Circulatory System. Not so conspicuous is the circulatory 

 system, which consists of two large blood vessels, one above, the 

 other below, the digestive tract, connected by branches in each 

 segment. Some of these branches pulsate, acting as a heart, to 

 drive the blood through the system. It must be remembered that 

 the functions of any circulatory system are ones of transportation. 

 It carries food from the digestive organs to the tissues, oxygen 

 from the breathing organs to the tissues, and waste products from 

 tissues to the organs of excretion. In all animals less specialized 

 than the worm, the structure was so simple that these processes 

 were carried on directly by osmosis, but in the worm, division of 

 labor is more complete, the various tissues more complicated and 

 so, for the first time, a transportation system is developed. 



Excretory and Nervous Systems. Besides the circulatory organs, 

 there are rather complicated sets of tubes in each segment, which 

 excrete waste matter. There are two sets of reproductive glands 

 between the pharynx and stomach. On the lower (ventral) side 

 of the body is a double row of light-colored threads (the nervous 

 system), united in each segment, and ending in a tiny knob near 

 the mouth, which corresponds somewhat to the brain. When 

 such an- animal is compared with the paramoecium, it is evident 

 that its functions have much better machinery for their perform- 

 ance. 



Locomotion. The worm is adapted for locomotion by the body 

 muscles and setae. The muscles extend the anterior part of the 

 body, the setae are slanted backward and grip the soil, and the 

 posterior part of the body is pulled forward with a sort of wave- 

 like motion. By this means the worm travels on the surface or 

 burrows in the ground. Burrowing is assisted by the fact that the 

 earthworm practically eats its way, taking the soil into its digestive 

 tract, absorbing what organic matter it can use as food, and bring- 

 ing the unused earth to the surface as " worm castings." These 

 are often seen on lawns, tennis courts, and golf greens. 



Analogous Organs. Organs in different animals which perform 

 similar functions are called analogous organs. The setae and 



