Final answer:
Planaria are freshwater flatworms with a protonephridia excretory system consisting of ciliated tubules, a gastrovascular cavity, and a simple nervous system. They are capable of regeneration and exhibit bilateral symmetry. While diverse in appearance, they are primarily non-parasitic and have organ systems.
Step-by-step explanation:
Planaria are a type of flatworms found in freshwater environments. One interesting aspect of these organisms is their excretory system, called protonephridia. This system consists of flame cells and a highly branched tube system, with tubules containing cilia, that propel waste out of the body through excretory pores. These cilia also help to draw water from the interstitial fluid, facilitating filtration and the reabsorption of valuable metabolites.
Planaria have a gastrovascular cavity with a single opening that serves both as a mouth and an anus. This flatworm's nervous system includes two interconnected nerve cords and cerebral ganglia at the anterior end, equipped with eyespots for photosensory and chemosensory functions. These characteristics are indicative of their bilateral symmetry and their status as a member of the animal kingdom with organ systems, despite their simple body plan.
Moreover, Planaria are known for their remarkable ability to regenerate lost body parts, which makes them subjects of scientific research. They exhibit significant diversity in their morphology, ranging in color and size, and can be either free-living or parasitic. In contrast to parasitic helminths, such as some roundworms and flatworms, Planaria are predominantly non-parasitic.