The cuticle is frequently prolonged into spines and papillae, which are especially developed at the anterior end of the body.
The Eolid-like Nudibranchs, amongst other specialities of structure, possess (in some cases at any rate) apertures at the apices of the " cerata " or dorsal papillae, which lead from the exterior into the hepatic caeca.
Some amongst them (Tergipes, Eolis) are also remarkable for possessing peculiarly modified cells placed in sacs (cnidosacs) at the apices of these same papillae, which resemble the " thread-cells " of the Coelentera.
Dorsum furnished on either side with papillae, at the base of which are ramified appendages.
Tegumentary papillae not ramified, and containing cnidosacs with nematocysts.