planàries

Planàries are a group of small, flatworms belonging to the class Turbellaria, primarily found in freshwater, marine, and terrestrial environments. They are soft-bodied, dorsoventrally flattened organisms that typically measure between 1 and 20 millimeters in length, though some species can grow larger. Planàries are known for their regenerative abilities, capable of regenerating entire bodies from small tissue fragments, including the head or tail.

These organisms possess a simple body structure with a bilateral symmetry, meaning their left and right sides are mirror images. Their bodies are covered by a thin, moist cuticle, allowing for efficient gas exchange. Planàries lack a circulatory system, relying instead on diffusion to transport nutrients and oxygen throughout their bodies. They move using cilia or muscular contractions, gliding smoothly across surfaces.

Planàries are carnivorous, feeding on small prey such as nematodes, rotifers, and other invertebrates. They use a muscular pharynx to suck in prey, which is then digested in an extensive gut. Some species are also known to exhibit complex behaviors, including learning and memory, as demonstrated in studies involving maze navigation.

Reproduction in planàries can occur both sexually and asexually. Many species are hermaphroditic, possessing both male and female reproductive organs, allowing them to cross-fertilize with other individuals. Asexual reproduction often occurs through transverse fission, where the worm splits into two parts, each regenerating the missing segments.

Planàries play important roles in aquatic ecosystems as both predators and prey. They are also of interest in scientific research due to their regenerative capabilities and simple nervous system, which make them useful models for studying development, regeneration, and neurobiology. Some species, such as *Dugesia* (formerly *Planaria*), are commonly used in laboratory settings for educational and experimental purposes.