Bases of the rhinophores surrounded by a sheath; dorsal papillae tuberculated and club-shaped, in a single row on either side of the dorsum; no cnidosacs.
It seems probable that it is identical with one of the open sacs in which each shell-plate of a Chiton is formed, and the series of plate-like imbrications which are placed behind the single shell-sac on the dorsum of the curious slug, Plectrophorus, suggest the possibility of the formation of a series of shellsacs on the back of that animal similar to those which we find in Chiton.
In this position the outer margin of the wing is the costa, the inner the dorsum, and the hind-margin the termen.
Respiration is effected by means of external gills placed along both sides of the dorsum of the abdomen and hinder segments of the thorax.
Both foreand hindwings are usually present, both pairs being membranous, the hindwings small and not folded when at rest, each provided along the costa with a row of curved hooks which catch on to a fold along the dorsum of the adjacent fore-wing during flight.
There are no fungiform papillae on the dorsum, but a few inconspicuous ones scattered along the sides of the organ.
Dorsum furnished on either side with papillae, at the base of which are ramified appendages.