any member of a group of transparent tunicates belonging to the class Appendicularia (subphylum Tunicata, phylum Chordata) that live in the open sea. The larvacean's tadpolelike body is made up of a trunk and tail and resembles the larval form of a sea squirt, a related form from the class Ascidiacea.

The body secretes a delicate, gelatinous house characterized by two openings located on opposite ends of the structure to enclose its trunk and body. The larvacean can propel the house forward through the water by beating its tail, which produces a unidirectional current that pulls water in through the forward opening of the house and pushes it out the rear. Microscopic food particles are captured from the water as they pass through the house. A larvacean abandons its house and secretes a new one several times each day.