172 



SEGMENTED WORMS— THE ANNELIDS 



into its interior, which is called a typhlosole. This makes the intestine 

 much more efficient for its length than it would be otherwise. 



The earthworm has a circulatory system, and one of the most strik- 

 ing things about this system is the presence of ten hearts. These hearts 

 are not nearly so complicated as the hearts of the vertebrate animals, 

 but they are organs which pump blood and this is the main function 

 of any heart. In addition to the hearts, there are two major blood 

 vessels with a number of smaller branches and capillaries which carry 

 the blood to all parts of the earthworm's body. The blood is red in 



Cuticle 



Epidermis 



Circular muscle 



Longitudinal muscle 

 Peritoneum 

 Coelom 

 Dorsal vessel 

 Chloragogue layer 

 Muscle layer 

 Epithelial layer 

 Typhlosole 

 Intestine 

 Nephridium 



— Ventral vessel 

 Nerve cord 



Setae 

 Giant fibers 



Fig. 12.2. Cross-section of the intestinal region of an earthworm. 



color due to the presence of hemoglobin which is the oxygen-absorbing 

 part of the blood, but microscopic examination reveals that the hemo- 

 globin is not contained in corpuscles as it is in the vertebrate animals, 

 but is dissolved in the plasma. 



If you put a living earthworm under a binocular microscope, or study 

 it with a hand lens, you will be able to see the blood flowing through the 

 dorsal vessel which lies near the dorsal surface. It moves in an anterior 

 direction and is carried along by peristaltic contractions of the vessel 

 which closely resemble the contractions of the alimentary tract previ- 

 ously described. This blood is on its way back to the hearts after a cir- 

 cuit around the body and there is little pressure left from the heart beats 

 so these contractions help it along. It flows down into the five pairs of 

 hearts which are nothing more than enlarged, contractile blood vessels 

 with valves at either end. These vessels allow the blood to flow in from 



