Carnivorous, feed on small animals and carrion. Lack specialized organs for gas exchange or circulation. Highly ramified gastrovascular cavity distributes food throughout the body. Flame cell excretory apparatus present which functions primarily to maintain osmotic balance of the animal. Move by using cilia on the ventral dermis to glide along a film of mucus.
Muscular contractions produce undulations which allow some to swim.
On the head are a pair of eyespots which detect light and a pair of lateral auricles that are olfactory sensors. Possess a rudimentary brain which is capable of simple learning. Reproduce either asexually or sexually. Sexual is by cross-fertilization of these hermaphroditic forms. All members of these two classes are parasitic.