THE EARTHWORM AND ANNELIDS IN GENERAL 175 



to be peristaltic, and have been likened to the action of the fingers 

 in the operation of milking. The valves in the walls of the dorsal 

 trunk prevent the return of blood from the anterior end. In 

 somites VII to XI the blood passes from the dorsal trunk into 

 the hearts, and is forced by them both forward and backward 

 in the ventral trunk. The valves in the heart also prevent the 

 backward flow. From the ventral trunk the blood passes to the 

 body wall and nephridia. Blood is returned from the body wall 

 to the lateral-neural trunks. The flow in the sub-neural trunk is 

 toward the posterior end, then upward through the parietal 

 vessels into the dorsal trunk. The anterior region receives blood 

 from the dorsal and ventral trunks. The blood which is carried 

 to the body wall and integument receives oxygen through the 

 cuticle, and is then returned to the dorsal trunk by way of the 

 sub-neural trunk and the intestinal connectives. Because of its 

 proximity to the sub-neural trunk, the nervous system receives a 

 continuous supply of the freshest blood (143, 146). 



Respiration. The earthworm possesses no respiratory sys- 

 tem, but obtains oxygen and gets rid of carbon dioxide through 

 the moist outer membrane. Many capillaries lie just beneath the 

 cuticle, making the exchange of gases easy. The oxygen is com- 

 bined with the haemoglobin. 



Excretory Organs. Most of the excretory matter is carried 

 outside of the body by a number of coiled tubes, termed nephridia 

 (Fig. 85, neph.), a pair of which are present in every somite except 

 the first three and the last. A nephridium occupies part of two 

 successive somites; in one is a ciliated funnel, the nephrostome 

 (Fig. 85, nephrost.), which is connected by a thin ciliated tube 

 with the major portion of the structure in the somite posterior 

 to it. Three loops make up the coiled portion of the nephridium. 

 The thin tube mentioned above is a single row of hollow cylin- 

 drical cells placed end to end; it extends through one loop and a 

 half, and connects with a larger tube brown in color and ciliated 

 throughout its central cavity. This portion, known as the middle 

 tube, opens into a third tube, which is wider, but without cilia. 



