Planarian

Planarian, a term practically co-extensive with Turbellarian, and applicable to the members of the lowest class (Turbellaria) of worm-like animals. They live in fresh and salt water and sometimes in damp earth. They are unjointed 'worms' with a ciliated skin; the food-canal is often branched, but has no posterior opening; from the simple brain two lateral nerves extend backwards; the body-cavity is undeveloped; there are no respiratory or circulatory organs; the excretory system consists of branching tubes ending in ciliated cells; all but two genera are hermaphrodite. Their simplicity is well illustrated by the fact that some multiply by dividing into two, while a fragment of others may re-grow the whole. In Microstoma lineare a temporary chain of eight or sixteen individuals is sometimes formed by budding. In diet they are carnivorous, but a few are parasitic—suggesting the next class of Trematodes. As illustrative genera we may note Planaria, in fresh water; Vortex and Convoluta, with green species (the colour being probably due to partner Algae); Gunda, with hints of segmentation; Microstoma and Stenostoma, the two unisexual genera; Graffilla and Anoplodium, parasitic; Bipalium and Geodesmus, on land; Cœloplana and Ctenoplana, in some ways suggestive of the Cœlenterate Ctenophora.