558 



EVOLUTION OF ANIMALS 



Part V 



titions of connective tissue. A layer of mesodermal cells, the peritoneum, 

 covers all surfaces facing the cavity, including those of the organs within it. 

 The cavity holds the watery coelomic fluid containing colorless amebfoid cells. 

 This is squeezed through pores in the septa and slowly circulated by move- 

 ments of the body. Some of it oozes through the dorsal pores and moistens the 

 body surface. 



Digestive Tract and Its Functions. Earthworms consume quantities of 

 bacteria and minute nematodes along with the soil that contains them. At 

 the surface they feed largely on plant fragments. A worm can be caught by a 

 spotlight and camera with a sheaf of broken leaves being sucked into its mouth. 

 In the pharynx leaves are moistened by slippery saUva and further broken 

 by squeezing (Fig. 28.6). 



The food then passes through the relatively long esophagus where it re- 

 ceives a milky fluid from the calciferous organs located in segments 10, 11, 

 and 12. All of these are outpocke tings of the wall of the esophagus. The first 



Segment I 



— Brain 

 Pharynx 



Aortic arcti I 

 Esoptiogus 

 Sem. ves. I 



Stom -int. 

 Dor, blood ves. 



Fig. 28.6. Earthworm. The general structure of the anterior part of the body 

 shown with the dorsal wall removed. The nephridia (kidneys) are not shown. 

 (Courtesy, Calkins: Biology. New York, Henry Holt and Co., 1914.) 



