Tethydids range in size from small to very large, and all have an elongate body. Their most distinctive feature is a very large, extensible oral hood with fringing tentacles that is used to trap prey. The rhinophores have very small, lamellate clubs. The rhinophoral sheaths, however, are quite large and cylindrical or flattened to be similar to the cerata. Each side of the body bears quite large cerata down the length. These are broad and flat except at the base where they are are more cylindrical for attachment. They are often fluid-filled but do not usually have branches of the digestive gland within. In some of the smaller species, the cerata may be tuberculate. The cerata can be shed when the animal is disturbed but they can regenerate. In most species, the bushy gills are scattered over the inside and outside surface of the cerata or over the whole dorsum. In others, pairs of gills may be attached to the base of the larger cerata. Some of the larger species can swim with lateral flexions of the body assisted by the large surface area of the erect cerata. This appears to be only an escape response and is in no way efficient. They feed upon small crustaceans captured in the oral hood that is held up in the current or cast over the substrate.