All organisms go through a life cycle that describes how they change from their origin to their ultimate end. Although the great majority of multicellular animals originate in the fertilization of an egg by sperm there are some exceptions, and the planarians are examples of this. Although new individuals can, and generally do, originate from fertilization these flatworms can also originate asexually by fission. Most planarians have simple life cycles.
In this mode of reproduction, the planaria simply constricts its body until it actually separates into two parts, one the anterior and the other the posterior ends. Each of the parts then regenerates the missing portion and thus two complete individuals arise. This mode of reproduction is rare, but it has been shown that it increases in frequency when the number of individuals in the area is low, possibly due to the difficulty in finding partners. Their capacity to regenerate is not restricted to reproduction, and if an investigator sections a planaria in two, each piece also generates a new individual.
Although planarians are hermaphroditic, they normally do not self-fertilize. This means that although an individual carries both ovaries and testes, it does not use its own sperm to fertilize its eggs. When mating, planarians exchange sperm and each individual is fertilized by the other. This mode of reproduction is the more common one and has the advantage of increasing the genetic variability of the species.
In some planarians, the yolk used to feed the embryo is not contained within female gamete but in specialized cells, called yolk cells, which are enclosed within the egg shell. Others follow the more traditional pattern in which the yolk is contained in the female gamete. Eggs are laid on the underside of rocks or vegetation, attached to the substrate by a short stalk.
In most planarians, the embryo emerges as a juvenile that resembles the adult but does not have functional gonads, which develop later. In some cases, especially in marine forms, embryos hatch as free swimming larvae that are very different from the adult and must undergo metamorphosis.
Planarians are free living, carnivore animals. This is in stark contrast with their closest relatives, tapeworms and flukes, which are parasitic. Planarians live in the sea, in fresh water and even on land, but in the latter case in very humid environments, away from sunlight. They can be as small as less than 5 mm or as big as 50 cm. They are flat and have very rudimentary sense organs and digestive systems.
