In terms of reproduction, rotifers are dioecious, but males are usually smaller with smaller organs or none at all.  Copulation involves hypodermic impregnation in which the male stabs the female in any part of her body and injects sperm that migrate to the eggs in the pseudocoel.  Parthenogenesis ("virgin birth")  is common in rotifers;  females produce diploid amictic ("without mixing") eggs by mitosis that develop into diploid females.  During unfavorable periods (such as crowding, drying, freezing in temporary ponds), females may produce haploid mictic eggs that if not fertilized, develop into males.  If these mictic eggs are fertilized, they become resistant eggs that can remain dormant and survive over the winter freeze, summer droughts, etc.  Another interesting feature of rotifers is their ability to enter a state of cryptobiosis ("hidden life").  Many species can endure long periods desiccation during which they shrink and enter a state of "suspended animation". During such periods they are extremely resistant to environmental extremes (they can even survive freezing in liquid helium).  By placing them in water again, they can recover even years later!