THE EARTHWORM 207 



The funnel-shaped end of a nephridium is provided with 

 cilia (hair-like structures) inside, which wave downward and 

 carry out such liquid waste products as collect in the body- 

 cavity. Blood-vessels, which break up into capillaries in the 

 wall of the nephridium, also carry a certain amount of waste. 

 The excess water and the nitrogenous waste in the blood 

 are separated in these capillaries ancTgo down the tube, to 

 be discharged at the external opening. The body-cavity fluid 

 is filled with small, free-moving cells, called amoebocytes. 

 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. The amcebocytes dissolve solid par- 

 ticles that fall from the superficial cells of the intestine. 

 Frequently they make their way through the entire intestine- 

 wall, and are passed to the exterior with the contents of the 

 alimentary canal, carrying the waste products with them. 



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 the earthworm if it 

 were not for the controlling influence of the nervous system. 

 Ingestion, digestion, absorption, circulation, respiration, oxi- 

 dation, and excretion constitute a chain of processes, largely 

 because 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 the earth- 

 worm may act, it will be necessary first to have some knowl- 

 edge of the structure of its nervous tissue. 



The general plan of the nervous system of the earthworm 

 is similar to that of the locust and the crayfish, but Less 

 specialization is evident than in either of the other animals. 

 The " brain" (supraoesophageal ganglion, Fig. 100, 27) is a 



