THE SIMPLEST LIVING THINGS 197 



cavity or series of cavities which, when distended, 

 contract and discharge their contents to the exterior. 

 This is an apparatus for " washing out " the protoplasm 

 of the unicellular animalcule and getting rid of excretory 

 products ; it is definitely comparable in its use, though 

 so different in origin, to the many-celled kidneys and 

 bladder of higher animals. 



One of the numerous kinds of "bell-animalcules" 

 affords an excellent example in which we can watch the 

 structures and life-processes in a single cell (Fig. 41). 

 It is a pear or bell-shaped body, little more than one- 

 thousandth of an inch broad, supported on a long, hollow 

 stalk (though sometimes it breaks off from its stalk and 

 swims freely) ; inside the stalk is a muscle (), so attached 

 that when it contracts it shortens the stalk by throwing 

 it into a close-set, corkscrew spiral (Fig. 4 IB). The 

 bell-shaped body has a relatively firm surface, beneath 

 which is soft, viscid protoplasm and a large sausage-like 

 nucleus. The body can expand itself so as to look like 

 a solid bell or trumpet-shaped figure, with a flat, disc- 

 like surface where the " hollow " of the bell should be, or 

 it can draw the edges of the disc together and assume 

 the shape of a ball. A line of " cilia " is set on the edge 

 of the bell's disc (a) and takes a spiral course. 



There is a deep pit on one side of the disc. This 

 is the mouth. It is easy to feed this minute " egg " of 

 a creature ! A powder of fine particles boiled bacteria, 

 in fact, are what I have used is introduced into the 

 water between two slips of glass in which the bell- 

 animalcule is displaying itself under our microsope. 

 We see the particles whirling about in a vortex, hitting 

 the disc of the bell-animalcule and then driven into 

 the pit or cavity of the open mouth, whence they 

 sink, enclosed in a sphere or droplet of water (f) into 

 the internal protoplasm ! If the " boiled bacteria," before 



