SUMMARYThe protective system of eolid nudibranchs consists of crypsis, defensive behaviour patterns, autotomy of cerata, exudation of glandular secretions, ejection of nematocysts, and the structure of the epidermis. The existence of warning colouration has not been established.Different protective mechanisms are adapted to perform different functions. Epidermal vesicles probably protect the animal from explosions of nematocysts whilst feeding, and many mucous glands perform a cleansing function. Other types of gland, nematocysts, autotomy of cerata, crypsis and behaviour are all mechanisms which are directed against predators and can therefore be regarded as defensive mechanisms.Concentrated batteries of glands are described from the ceras tips of species of Catriona and of the Eubranchidae. Some of these are mucous glands, others contain protein secretions. There is evidence that these glands are important in the defence of the eolid.Nematocysts and the various types of defensive gland are probably adapted towards certain specific predators since a single mechanism is unlikely to be effective against all predators. Since the predators of different species of eolids vary, there is variation in the development of certain defensive mechanisms in these eolids. In the Aeolidiidae, Tergipes despedus, Selva rubra, Palisa papillata and perhaps in some other cleioprocts, particular types of nematocysts are stored in large numbers and these explode readily when ejected. Species of Catriona and the Eubranchidae also use nematocysts, but in most of them the concentrated batteries of glands at the ceras tip are of more importance than are the nematocysts. In Calma glaucoides nematocysts are absent, and similar batteries of glands are concentrated towards the ceras tip.In Catriona aurantia, and probably in all eolids, exudation of defensive glands and ejection of nematocysts is caused by stimulation of special neurosensory cilia which are concentrated at the tips of the cerata. Nematocysts are ejected by contraction of muscles in the cnidosac wall, and some of the defensive glands in the Eubranchidae and in Catriona species are muscle‐operated.The defensive mechanisms of eolids are concentrated in the cerata. The colour of the cerata is largely responsible for crypsis; defensive behaviour and autotomy both involve the cerata; and defensive glands and nematocysts are concentrated at the tips of the cerata. It is therefore probable that the most important function of the cerata is that of protecting the eolid against predatio