Sponges, sea anemones, tapeworms, snails, and earthworms are all simultaneous hermaphrodites possessing both male and female reproductive organs at the same time. These animals are either sedentary species (remaining in one place all their life) or they are mobile, but do not range widely. These habits present problems for sexual reproduction and mating, for individuals rarely meet others of their own species. When two simultaneous hermaphrodites, such as two slugs, meet and mate, each one can fertilize the eggs of the other.
A common misconception about hermaphroditic reproduction is that organisms fertilize their own eggs with their own sperm. In fact, most species do not self-fertilize, and many are physically incapable of self-fertilization. (Bisexual plants are the exception, and some do self-fertilize.) For example, sponges fertilize externally, but they release eggs and sperm into the plankton at different times, so that gametes encounter, and are likely to be fertilized by, or to fertilize, gametes from other individuals. The reproductive organs of earthworms are positioned at different ends of their bodies, so that fertilization of the eggs is only possible when the worms are aligned in opposite directions. Some simultaneous hermaphrodites even have alternative reproductive methods. The larvae of digenetic trematodes, such as the liver fluke, are simultaneous hermaphrodites, and reproduce asexually, while the adults reproduce sexually.