AN EARTHWORM 243 



excretory organs are called nephridia. They correspond in 

 function to the kidneys of* the higher animals. Each ne- 

 phridium opens into the cavity immediately anterior to the 

 somite which contains the greater portion of the organ. The 

 external opening of a nephridium is on the ventral surface. 



The funnel-shaped end of a nephridium is lined with cilia 

 (hair-like structures). The cilia beat inward and carry such 

 liquid waste products as collect in the body cavity into the 

 tube of the nephridium. Blood vessels, which break up into 

 capillaries in the wall of the nephridium, also carry a cer- 

 tain amount of waste. The excess water and the nitrogenous 

 waste in the blood are separated in these capillaries and go 

 down the tube, to be discharged at the external opening. 

 The body-cavity fluid is filled with small, free-moving 

 cells, called amcebocytes. These amcebocytes are similar to 

 the white corpuscles of our own blood. They have the 

 power of changing their form quickly by extending irregular 

 pointed processes from any part of the cell body. Owing to 

 this power they can inclose any small particle of solid matter. 



Nerve Control. In the preceding paragraphs we have been 

 considering the stages of metabolism, without making any 

 reference to the fact that no activity of internal or external 

 organs could take place in the body of an earthworm if it 

 were not for the controlling influence of the nervous system. 

 Ingestion, digestion, absorption, circulation, respiration, 

 and excretion constitute a chain of processes, largely be- 

 cause the nervous system, acting through the system of 

 muscles, especially in the digestive and the circulatory sys- 

 tems, causes them to take place according to a definite plan. 

 In order to understand how the nervous system of an earth- 

 worm acts, it will be necessary first to have some knowledge 

 of the structure of its nervous tissue. 



The general plan of the nervous system of an earthworm 

 is similar to that of the grasshopper and the crayfish, but 



