TISSUE-DIFFERENTIATION 1 19 



the outer margin of the cell-body toward which the 

 dissolved " waste-products " concentrate. These 

 form a gradually enlarging bubble which finally 

 ruptures and squeezes its contents out through the 

 cell- wall to the exterior. In most Protozoa this 

 excretory function is definitely localized, and the 

 formation and extrusion of the excreted drop of 

 fluid follows a regular rhythm. Such an organ is 

 called a " contractile vacuole." With the enlarge- 

 ment of the body and the multiplication of the cellu- 

 lar units composing it, it becomes impossible for such 

 excreta to find their way to any particular spot, or 

 even to diffuse through the tissues with sufficient 

 rapidity to prevent the ill effects mentioned above 

 (" intoxication "). Definite channels are found in 

 most Metazoa through which these products pass. 

 In the tapeworm and its allies these tubes are very 

 delicate and permeate the body in all directions, 

 opening to the exterior in pores. Each tube is 

 thought to be the differentiation of a single cell and 

 each terminates (or rather, more accurately, origi- 

 nates) in a so-called " flame-cell." This cell has a 

 wisp of cilia projecting into the cavity of the tube, 

 by the flickering movement of which, suggestive of a 

 candle flame, the absorbed fluids are urged down the 

 tube and ultimately to the exterior. 



In the great group of the Annelids, of which the 

 earthworm is the most familiar example, the body is 

 composed of a great number of segments arranged 

 like a row of pill-boxes strung on a tube, the tube 

 being the alimentary canal. Each segment, al- 



