Planarias

Planarias, or planarians, are free-living flatworms in the phylum Platyhelminthes. They are most diverse in freshwater habitats but can also be found in marine environments and damp terrestrial habitats. They range in size from a few millimeters to several centimeters and have a soft, elongated, flattened body.

Their anatomy features a relatively simple organization: a ventral mouth opening leading to a branched gastrovascular cavity, with three main intestinal branches typical of triclad planarians. The nervous system is a ladder-like arrangement of bilobed cerebral ganglia connected to longitudinal nerve cords. They possess two eyespots (ocelli) on the head and sensory auricles that help detect chemicals and light. The body surface is ciliated, and locomotion is achieved by ciliary gliding on a film of mucus.

Planarians are famous for regeneration. They possess neoblasts, pluripotent stem cells distributed throughout the body, which drive tissue renewal and regrowth. When injured or reduced to fragments, planarians can regenerate missing organs and even reconstruct a complete animal, sometimes within days to weeks depending on species and fragment size.

Reproduction is versatile. Many species are hermaphroditic and can reproduce sexually via cross-fertilization, producing cocoons. They also reproduce asexually by transverse fission or fragmentation, especially in stable freshwater environments.

Ecology and laboratory research: in their natural habitats, planarians feed on small invertebrates and detritus, using a protrusible pharynx to ingest prey. In science, they are widely used as model organisms to study stem cells, regeneration, and developmental biology due to their robust regenerative capacity and simple body plan.