Elongate and usually high bodied nudibranchs with a tough body wall thickened with spicules making some inflexible and very rigid to the touch. In some species there is a complete loss of the mantle skirt and they possess small rounded or irregular tubercles studded over the body.
In others, the mantle is reduced to a minimal anterior ridge plus a row of prominent tubercles around the edge of the body and similar tubercles over the rest of the body. The rhinophores are smooth and can retract into either raised sheaths or pockets that have covering flaps. The gills and their protective structures are of two types.
They can be bipinnate or tripinnate and protected by a substantial anterior tubercular transverse ridge, or else simple or tripinnate and surrounded by a group of protective tubercles.
All aegirids feed upon calcareous sponges and all are slow movers.