The general body shape may be slender or stout with length ranging from 5–35 mm. Body colour of the hermaeids can vary from transparent and colourless to heavily pigmented. The rhinophores are auriculate with a characteristic and diagnostic distinct step towards the tip. Eye spots are located behind the rhinophores.
Oral lobes may be present extending laterally above the mouth. The presence of the pericardium is indicated by a swelling immediately behind the neck. The anus opens either through a long posteriorly-directed papilla or is flush with the neck.
The male pore is behind, and below, the right rhinophore with the single female pore just posterior to that. Cerata shape vary from short, to long-fusiform, or even long paddle-shaped with branches of the digestive gland visible inside.
Many possess only a single row of cerata down each side alternately large and small.
Hermaeids feed upon algae.