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.
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.
A, of Helix; B, of Eolis; a, ova; b, developing spermatozoa; c, common efferent duct.
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