The opisthobranch breathing is done with unique organs for this group of animals called cerata and branchial plumes or gills.
The cerata are structures created as a finger or stick, arranged in uniform groups along each side of the mantle of the animal. They are present in all aeolidacean and dentronotacean nudibranchs and in some arminaceans and sacoglossans. Its function is digestion, respiration and defense. Histologically, the tissue of the cerata is no different from the rest of the body. In most groups, the cerata usually contain branches of the digestive gland. The doridacean nudibranchs often have a circle of gills surrounding the anus, used to breathe, and while some doridaceans may have some sort of projections on the back, they are not called cerata, nor known to have any respiratory functions.
The gills in cephalaspideans, anaspideans and notaspideans are more developed, usually located inside the mantle cavity, or below the notum.
In doridacean nudibranchs we can differentiate three types of gills (unipinnate, shielded and tripinnate). In cryptobranchs gills can be withdraw within a sheath and within the body, like the rinophores, for protection. Phanerobranchs lack a sheath for the gills to be hidden. Phyllidiidae family members do not have gills surrounding the anus but, instead, have a number of folds in the form of gills on both sides of the body, between the notum and the foot.
In some nudibranchs, all cerata contain a terminal cnidosac with defensive purposes, since the animal keeps in it active nematocyst capsules (stinging harpoon-shaped cells). These tiny cells, captured intact from their food, pass through the entire digestive system of the animal and move to the cerata, where they are stored, still active, for defense purposes.
The cerata can be autotomized or cast-off by the animal, and then regenerated quickly. In some species the cerata are detached at the slightest provocation, it is believed to be a kind of diversion strategy similar to lizards that come off their tail when attacked. When autotomy takes place there is no loss of material from the digestive tract as the insertion point of the cerata is sealed immediately. The cerata is completely regenerated in a few days.
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