Mode of reproduction in planaria

A planarian is one of many flatworms of the Turbellaria class.[1] It is also the common name for a member of the genus Planaria within the family Planariidae. Sometimes it also refers to the genus Dugesia.[2]

Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found under logs, in or on the soil, and on plants in humid areas.

<a id="full-answer">Reproduction</a>

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.