One of the most remarkable characteristics of opisthobranchs is their coloration, which plays an important defensive role in many species. Many opisthobranchs, especially the most evolved, have bright, glowing colors that are normally species-specific, though coloration can vary drastically between closely related species. In most of these cases they obtain carotenoid pigments through food and modify them chemically to alter their coloration. Others are cryptically colored. Color pattern may also change during the development of some species.

Adult opisthobranchs display an evolutionary tendency towards the reduction or complete loss of the shell. However, the most primitive forms have well-calcified shells into which the animal may completely contract its soft parts like terrestrial snails. And , all opisthobranchs have a shell in their larval stage. This is generally very small and simple.

Papillae on the dorsum of Cyerce orteai

Groups of opisthobranchs with internal shells or without shells have developed a noticeable dorsal structure which protects the internal organs. The backs of many also have "papillae," or prolongations, of many types and forms. They may be branched or simple, smooth, or in the form of a finger. Papillae may or may not house extensions of the digestive gland.

Some have cephalic shields adapted to penetrate soft substrates. Others have different types of sensorial structures such as bristles, oral tentacles, and rhinophores, a pair of specialized tentacles located on the head, that assume different positions and forms according to group.

Their gills, which are used for gas interchange, are located either in the cavity of the mantle, or externally on the right side of the body or on the center of the back. In some groups this consists of only a single branchial leaf, while in others the gill is composed of a ring of branchial leaves. In some lineages, the gill has been lost and gas interchange is accomplished through secondary respiratory structures or even the surface of the body.