Planaria are able to reproduce either sexually or asexually, depending on the species and the circumstances of reproduction. Sexually reproducing planaria are hermaphrodites, and mating consists of partners exchanging sperm with each other before departing to lay eggs.
Despite the similarity of planarian sexual reproduction to that of other animals, planaria are capable of reproducing asexually by binary fission. This mechanism takes advantage of the planarians' extreme facility to regenerate lost sections of their bodies. Once a planarian is split in half – a division that can take place along any axis of its body: latitudinal, longitudinal or coronal – each section of the body activates special cells called neoblasts. Neoblasts are adult stem cells that can divide into new cell lineages that later specialize into all the body's tissues. Neoblasts at the site of the break begin generating new tissue to replace the structures each half has lost, which results in two new flatworms.
This process of reproducing via whole-body division can happen as a result of traumatic injury, or it can be initiated by the planarian itself as a normal process called transverse fission. When the planarian initiates the process, its body splits latitudinally between head and tail sections.