214 CIECTJLATION IN THE LOWEE ANIMALS 



the tube to its starting point. The heart is simply a 

 contractile structure whose office is to keep the liquid in 

 motion. 



The earthworm. — In this animal we have a slight 

 advance over the insect. It possesses a true closed sys- 

 tem of tubes conveying a red blood. It consists of two 

 tubes running lengthwise of the body, one dorsal and 

 one ventral. The dorsal blood vessel can be easily made 

 out through the transparent skin of the living earthworm, 

 and observation shows that the red liquid which it con- 

 tains moves in pulses toward the head. At the anterior 



Fig. 78 — Diagram of a longitudinal section of an earthworm ; ft, one of the fine 

 pulsating arches which pump hlood from the dorsal blood vessel into th6 ventral 

 blood vessel. 



end of the animal are five pairs of arches or blood vessels, 

 which branch from each side of the dorsal blood vessel 

 and, encircling the esophagus, open below it into the ven- 

 tral blood vessel. These pairs are arranged, one behind 

 the other, hke rings from the sixth to the eleventh segment. 

 These five pairs of tubes are called the aortic arches, 

 and they pulsate in unison, forcing the blood to flow 

 from the dorsal blood vessel into the ventral. The course 

 of the blood, then, is from the rear of the body along the 

 dorsal blood vessel to the head; downward, through the 

 arches to the ventral vessel, and through' this toward the 

 rear of the body again. Both dorsal and ventral blood 



