They are usually easily recognized by the plump body shape with a full and rounded dorsum. There is no external shell but an internal remnant may be present.
The mantle may be smooth or evenly covered with rounded pustules. Glands in the skin secrete acidic fluid for defensive purposes. A broad oral veil lies above the mouth formed between the oral tentacles. Both the oral tentacles and rhinophores are enrolled by virtue of a longitudinal slit.
Rhinophores may be carried close together on top of the head or spread to each side.
The plume-like gill is located on the right side between the mantle and the foot. Locomotion is by cilia on the foot assisted by mucus from an anterior pedal groove. Their diet varies among species and ranges from sponges to corals and anemones to ascidians. They feed through a protrusible oral tube.
Most often seen at night or found under rocks or coral plates during the day.