WHEEL ANIMALCULES 



up any delicate alga or protozoan which comes their way. 

 In the plant-eating rotifers the mastax works like a' suction 

 pump; the rotifer pierces the cell with its jaws and then 

 pumps it out clean. Cilia and mastax are equally char- 

 acteristic of rotifers and both keep moving as long as the 

 animal lives. On the moving rotifer the "foot" projects out 

 from behind and would be far better named the tail; it secretes 

 the glue by which the rotifer attaches itself to plants or often 

 to the backs of little crustaceans. Only female rotifers are 

 ordinarily seen and only in them is the characteristic mastax 

 present (Fig. loi). Males are extremely minute; in some 

 species of rotifers the eggs develop without fertilization. 

 Males have not been found and it is pretty certain that they 

 do not exist at all. 



Habits, resistance to cold and drought. — Rotifers can live 

 through great extremes of cold and drought. In the fall they 

 produce tough shells and lie dormant in them through the 

 winter months, hatching in spring into the individuals which 

 carry on the race. Some rotifers, Philodinidce (Fig. lOO, i), 

 are dried up, often through long droughts, but are able to" 

 keep life going at such a low rate that they can survive and 

 resume their normal living after the crisis is over. They 

 can also regain their activity after being frozen into ice for 

 long periods. 



Fig. 102. — The case-building rotifer, Melicerta: i, 

 animal and case; 2, a cluster of cases on lily pad. 

 Visible with hand-lens. ( i , from Ward and Whipple.) 



129 



