Nudibranch

A nudibranch is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms. The clade Nudibranchia is the largest clade within the heterobranchs, with more than 3,000 described species.

Nudibranchs live at virtually all depths of salt water, but reach their greatest size and variation in warm, shallow waters.

The body forms of nudibranchs vary a great deal, but because they are opisthobranchs, unlike most other gastropods they are bilaterally symmetrical both externally and internally because they have undergone secondary detorsion. They lack a mantle cavity. Some species have venomous appendages (cerata) on their sides which are used to deter predators. Many also have a simple gut and a mouth with a radula. 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. The adult form is without a shell or operculum (in shelled gastropods the operculum is a bony or horny plate that can cover the opening of the shell when the body is withdrawn).

Source: en.wikipedia.org/wiki/Nudibranch - 20.11.2008