176 GEXERAL BIOLOGY 



muscle and connective tissue 



blood and blood vessels 



peritoneum and chloragogue 



nephridia 



reproductive organs 

 Let us now try to get a bird's eye view of the life process 

 in the worm. Both the substance for the building of its 

 body and the energy for its operation are contained in the 

 food. This consists of organic matter (proteins and carbo- 

 hydrates) and salt solutions mixed with a remarkable large 

 proportion of indigestible materials (sand, clay, etc.), in the 

 earth and rubbish that the worm swallows : also, of the free 

 oxygen absorbed through the skin from the air. The solid 

 food is pulled to pieces by the prostomium, sucked in by the 

 pharynx, passed down the slender esophagus to a 

 temporary storage in the crop and triturated by the gizzard 

 (all purely mechanical treatment), perhaps mixed with 

 some secretions along the way, and then passed on into the 

 stomach-intestine. Here digestion and absorption take 

 place. These are the work of the epithelial cells. But 

 these cells are remote from many parts of the body, and all 

 parts have to be fed, therefore, the food must be trans- 

 ported. Circulatory apparatus exists because the slow 

 process of diffusion is inadequate to the needs of so large and 

 active an organism. Into the blood percolating through 

 the intercellular spaces about the bases of the epithelial cells 

 the dissolved food diffuses, and passes upward through the 

 dorso-intestinal vessels to enter the great dorsal trunk for 

 distribution all over the body to every living cell. 



For nutrition is at bottom the work of the cell. One set 

 of cells may attend to the digesting and another to the cir- 

 culating of the digested material, but every cell must eat for 

 itself. No amount of division of labor can relieve any cell 

 of the necessity of assimilating and excreting. For these 



