118 



GENERAL BIOLOGY 



nated as soon after their formation as possible. Even 

 in the least differentiated of the Protozoa special 



FIG. 44. Diagram of various types of excretory organs: A, Par- 

 amecium with two pulsating vacuoles, the upper one is at the close of 

 cycle, the lower one at the beginning ; B, flame-cell of a "flatworm" ; 

 fl, the wisp of cilia, the vibration of which urges the excreted matter 

 down the duct ; n, nucleus ; gr, granules of waste products ; C, simple 

 nephridium of Polygordius ; d, septum ; st, nephrostome or ciliated 

 funnel ; op, external orifice ; D, nephridium of a highly specialized 

 annelid worm, the blood vessels in black ; E, nephridial element of 

 vertebrate kidney ; g, glomerulus ; cap, capsule of urinary tubule 

 ciliated in its lower portion, blood-vessels in black. 



structures are found which provide for such elimina- 

 tion. In such organisms there is usually a spot on 



