Where the modification is carried to its extreme degree, not only the shell but the pallial cavity, ctenidium and visceral hump disappear, and the body acquires a simple elongated form and a secondary external symmetry, as in Pterotrachaea and in Doris, Eolis, and other Nudibranchia.
A, of Helix; B, of Eolis; a, ova; b, developing spermatozoa; c, common efferent duct.
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.
A, Eolis papillosa (Lin.), dorsal view.
- Enteric Canal of Eolis papillosa.