Planarians (triclads) are free-living flatworms of the class Turbellaria,[2][3] order Tricladida,[4] which includes hundreds of species, found in freshwater, marine, and terrestrial habitats.

Also, flame cells remove unwanted liquids from the body by passing them through ducts which lead to excretory pores, where waste is released on the dorsal surface of the planarian.

[5] The brain has been shown to exhibit spontaneous electrophysiological oscillations,[16] similar to the electroencephalographic (EEG) activity of other animals.

In asexual reproduction, the planarian fissions and each fragment regenerates its missing tissues, generating complete anatomy and restoring functions.

[19] In most of the cases the sexual reproduction involve two individuals; auto fecundation has been rarely reported (e.g. in Cura foremanii).

[23] The life history of planarians make them a model system for investigating a number of biological processes, many of which may have implications for human health and disease.

Like other invertebrate model organisms, for example C. elegans and D. melanogaster, the relative simplicity of planarians facilitates experimental study.

Thomas Hunt Morgan was responsible for some of the first systematic studies (that still underpin modern research) before the advent of molecular biology as a discipline.

These animals have an apparently limitless regenerative capacity, and asexual Schmidtea mediterranea has been shown to maintain its telomere length through regeneration.

[24] Live planarians are increasingly used in toxicological research due to their regenerative capabilities, simple anatomy, and sensitivity to environmental changes.

Their ability to regenerate lost body parts provides a unique model to study the effects of chemical exposures on cellular processes, while their rapid response to toxins makes them an efficient tool for screening potential environmental and pharmaceutical hazards.

[26] Neoblasts localized near the injury site proliferate to generate a structure of differentiating cells called blastema.

[28] Many signaling molecules that provide positional information to neoblasts, in regeneration and homeostasis, are expressed in muscle cells.

[29] Following injury, muscle cells throughout the body can alter the expression of genes that encode molecules that provide positional information.

In addition, existing tissue is remodeled to restore symmetry and proportion of the new planaria that forms from a piece of a cut up organism.

[35] Researchers, including those from Tufts University in the U.S., sought to determine how microgravity and micro-geomagnetic fields would affect the growth and regeneration of planarian flatworms, Dugesia japonica.

Two-headed planaria regenerates can be induced by treating amputated fragments with pharmacological agents that alter levels of calcium, cyclic AMP, and protein kinase C activity in cells,[37] as well as by genetic expression blocks (interference RNA) to the canonical Wnt/β-Catenin signalling pathway.

[28] In 1955, Robert Thompson and James V. McConnell conditioned planarian flatworms by pairing a bright light with an electric shock.

Subsequent explanations of this scrunching behaviour associated with cannibalism of trained planarian worms were that the untrained flatworms were only following tracks left on the dirty glassware rather than absorbing the memory of their fodder.

In 2012, Tal Shomrat and Michael Levin have shown that planarians exhibit evidence of long-term memory retrieval after regenerating a new head.

In recent decades, S. mediterranea has emerged as the species of choice for modern molecular biology research, due to its diploid chromosomes and the availability of both asexual and sexual strains.