PHYLUM ANNELIDA 223 



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. 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 subneural 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 subneural 

 trunk and the intestinal connectives. Because of its proximity 

 to the subneural trunk, the nervous system receives a continu- 

 ous supply of the freshest blood. 



RESPIRATION. - - The earthworm possesses no respiratory 

 system, 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 combined with the haemoglobin. 



EXCRETION. - - Most of the excretory matter is carried out- 

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

 (Fig. 153, neplr), 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 nephro- 

 stome (Fig. 153, nephrosi), 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 cilia on the nephrostome and in the nephrid- 

 ium create a current which draws solid waste particles from 

 the ccelomic fluid. Glands in the coiled tube take waste matter 

 from the blood, and the current in the tube carries it out through 

 the nephridiopore (ext.nepJi). 



NERVOUS SYSTEM. - - The nervous system differs from that of 

 the types studied heretofore in being more concentrated. There 

 is a bilobed mass of nervous tissue, the brain or suprapharyngeal 



