planaire

Planarian, or planaire, refers to free-living flatworms within the phylum Platyhelminthes, class Turbellaria, and order Tricladida. The term is most often applied to freshwater species, though some marine and terrestrial planarians exist. They are small to several centimeters in length, with a soft, flattened body. A characteristic feature of many Tricladida is a branched gastrovascular system and a ventral pharynx used for feeding.

Anatomy and physiology: Planarians possess a relatively simple nervous system that includes a bilobed brain (cephalic ganglion) and two ventral nerve cords linked by transverse connections. They have light-sensitive eyespots and rely on chemical and tactile cues to navigate. Gas exchange occurs across the body surface aided by a mucus layer and cilia, with no specialized respiratory or circulatory organs. The digestive system consists of a central gastrovascular cavity with branches extending into the body.

Regeneration and reproduction: Planarians are renowned for their regenerative abilities. They harbor pluripotent stem cells called neoblasts that can differentiate into all cell types, allowing complete regeneration from tissue fragments. Reproduction is typically hermaphroditic, with cross-fertilization in sexual reproduction; many species can also reproduce asexually by fission or fragmentation.

Ecology and significance: Freshwater planarians inhabit ponds, streams, and wetlands, feeding on small invertebrates or detritus. Some terrestrial species are predatory and travel as flat, sliding organisms across damp soil. Planarians are important model organisms in regenerative biology and developmental genetics, with notable species including Schmidtea mediterranea and Dugesia japonica.