A collection of nudibranchs seen in Lembeh.
The body forms of nudibranchs vary enormously, but because they are opisthobranchs, unlike most other gastropods they are bilaterally symmetrical because they have undergone secondary detorsion. Some species have venomous appendages on their sides. These are used to deter predators. Many also have a simple gut and a mouth with a radula.
They lack a mantle cavity.
Their eyes are simple and able to discern little more than light and dark. The eyes are set into the body, are about a quarter of a millimeter in diameter, and consist of a lens and five photoreceptors.
They vary in adult size from 20 to 600 millimetres (0.79 to 24 in).
The adult form is without a shell or operculum (a bony or horny plate covering the opening of the shell, when the body is withdrawn).
The name nudibranch is appropriate, since the dorids (infraclass Anthobranchia) breathe through a “naked gill shaped” like branchial plumes of bushy extremities on their back, near their tail rather than using gills. By contrast, on the back of the aeolids in the clade Cladobranchia there are brightly colored sets of protruding organs called cerata.
Nudibranchs have cephalic (head) tentacles, which are sensitive to touch, taste, and smell. Club-shaped rhinophores detect odours
