16 CELLS AND THEIR PARTS 



be at the hinder end of the cell, in which case it pushes the 

 cell forward like a proj^eller in the stern of a boat. But much 

 more commonly each cell has a group of cilia which wave to 

 and fro, as the stems in a field of corn wave when swept by 

 a Avind and right themselves when the wind has passed. Such 

 flickerings row an isolated cell along like the rhythmic beat 

 of the oars in a boat. 



In higher multi-cellular bodies, like our own, cilia are used 

 to push bodies along. They move to and fro, creating a 

 current of fluid or mucus, and in this current particles are 

 swept forward. In the human body the air passages are lined 

 mth cilia, and these cilia drive bacteria and other foreign 

 bodies up towards the mouth and so in time out of the body. 

 They also line the oviducts, and help to push the human egg 

 down into the uterus. Others have been found in the testis, 

 and again ciliated cells line the cavity of the brain and the 

 minute central canal which passes do^vn the spinal cord. 

 The cilia which move the spermatozoa in search of the 

 ova may also be regarded as flagella. In the gullet of the 

 frog cilia work downwards and help the creature to swallow. 

 In many aquatic animals, such as the oyster and the mussel, 

 which are either fixed or very sluggish, they keep the body 

 supplied with fresh volumes of water which bring food to 

 the mouth of the mollusc and oxygen to its gills, and the 

 same holds good in many other fixed, sessile, aquatic animals. 

 Some members of the freshwater animals, known as Roti- 

 fers or wheel-animalcules, do not move about, and here cilia 

 create a current which is the sole source of the food-supply 

 of these curious little creatures. 



The cilia of an isolated cell from a frog's gullet, floating 

 in salt solution and com^^letely removed from all nervous 

 control, will continue to beat rhythmically for many hours, 

 yet in the body of the frog their beat maybe slowed down 

 or completely stopped by various stimuli. The beating of 

 cilia from the mantle of certain molluscs stops at once when 

 stimulated by an electric current; however, if the mantle is 

 first treated with an anaesthetic such as novocaine, the electric 

 stimulus produces no effect and the cilia continue to beat. It 

 is believed that the first, or primary, inhibition, or stopping 



