Larvaceans, copelates or appendicularians, class Appendicularia, are solitary, free-swimming tunicates found throughout the world's oceans. While larvaceans are filter feeders like most other tunicates, they keep their tadpole-like shape as adults, with the notochord running through the tail. They can be found in the pelagic zone, specifically in the photic zone, or sometimes deeper. They are transparent planktonic animals, usually ranging from 2 mm (0.079 in) to 8 mm (0.31 in) in body length including the tail, although giant larvaceans can reach up to 10 cm (3.9 in) in length.

Larvaceans are known for the large houses they build around their bodies to assist in filter-feeding. Secreted from mucus and cellulose, these structures often comprise several layers of filters and can reach up to ten times their body length. In some genera like Oikopleura, houses are built and discarded every few hours, with sinking houses playing a key role in the oceanic carbon cycle.

The study of larvaceans began with the description of Appendicularia flagellum by Chamisso and Eysenhardt in 1821. More species were quickly discovered, with Oikopleura in 1830 providing the first evidence of the larvacean house, although its role in feeding wouldn't be understood until Eisen's discoveries in 1874.

Huxley was the first to suggest the identity of larvaceans as tunicates in 1851. Their relationship with other tunicates remained unclear, with larvaceans being argued to be ascidian larvae or a free-swimming generation of ascidians.

An attempt at establishing the internal phylogeny of the class was realized by Fol following the discovery of the aberrant Kowalevskia. Fol grouped together the families Oikopleuridae and Fritillariidae in the putative Endostyla, based on the presence of an endostyle, absent in Kowalevskia which he placed in the sister group Anendostyla.

Another jump in the study of larvaceans was the beginning of in situ observations, which allowed researchers to study the creatures inside their fragile houses without damage. Researchers such as Kakani Katija Young from the Monterey Bay Aquarium Research Institute pioneered imaging techniques such as the particle image velocimetry instrument DeepPIV, revealing the complexity and inner structure of larvacean houses and leading to the first 3D simulations of their internal currents.

The adult larvaceans resemble the tadpole-like larvae of most tunicates. Like a common tunicate larva, the adult Appendicularia have a discrete trunk and tail. It was originally believed that larvaceans were neotenic tunicates, giving them their common name. Recent studies hint at an earlier divergence, with ascidians having developed their sessile adult form later on.

As the larvae of ascidian tunicates don't feed at all, the larvae of doliolids goes through their metamorphosis while still inside the egg, and salps and pyrosomes have both lost the larval stage, it makes the larvaceans the only tunicates that feed and have fully functional internal organs during their tailed "tadpole stage", which in Appendicularia is permanent.