458 The Animal Kingdom 



the smaller ventral blood vessel by a series of five pairs of hearts which 

 pass around the digestive tract in segments 7 to 11. The pulsating dor- 

 sal blood vessel carries the blood forward to the hearts and to the anterior 

 portion of the body. The ventral blood vessel is located between the 

 digestive tract and the ventral nerve cord. It delivers blood to the 

 nephridia, the body wall, and the ventral wall of the intestine through 

 a series of smaller blood vessels Ventral to the nerve cord is the 

 subneural blood vessel which receives blood from two lateral neural 

 blood vessels. These latter in turn receive blood from the ventral body 

 wall. These three blood vessels plus the ventral blood vessel (after 

 segment 11) all carry blood toward the posterior portion of the body. 

 Blood is carried dorsad to the dorsal blood vessel through a series of 

 parietals which arise from the subneural blood vessel and others from 



HEART_ 



VENTRAL 

 VESSEL 



LATERAL 

 NEURAL 

 VESSEL 



SUBNEURAL 

 VESSEL 



.ESOPHAGEAL 



NERVE CORD 



Fig. 149. — Diagram of the circulation of tlie blood in the earthworm. 

 Arrows indicate the direction of flow. 



the digestive tract. There are several other blood vessels which carry 

 blood into the typhlosole, to the nephridia, and the skin surface. The 

 blood is pumped forward by peristaltic waves in the dorsal blood vessel ; 

 valves within the vessel prevent backward flow. The blood of the earth- 

 worm is red in color due to the dissolved hemoglobin. Colorless amoe- 

 bocytes circulate in the fluid. 



There is no specialized respiratory system ; gaseous exchange 

 occurs through the capillaries lying within the body wall. All segments 

 except the first three and the last contain a pair of coiled nephridia. 

 Each nephridium consists of a ciliated nephrostome and the coiled tube 

 which opens through the nephridiopore. The nephrostome lies at the 

 anterior part of a septum in the region of the nerve cord. The tube 



