The nudibranchs Aeolidia papillosa (Linnaeus, 1759) and Aeolidiella glauca (Alder and Hancock, 1845) were fed with the sea anemones Metridium senile (Linnaeus, 1759) and the small anemone Sagartiogeton viduatus (Muller, 1776). Aeolid nudibranchs are known for storing nematocysts for self-defence from cnidarian preys in their numerous dorsal papillae or cerata. The nudibranchs possess a specific sorting mechanism to sequester these nematocysts. In each cerata tip a divertriculum of the gut opens into a cnidosac. Unfired nematocysts are stored in cnidophage cells inside the cnidosacs. One, two, three or more anemone nematocysts, of the same or of different types, could be present inside one cnidophage. Cnidophages, filled with nematocysts, were pressed out through the cnidosac pore. A. glauca and A. papillosa seemed to have a certain preference to store the most potent, large penetrating nematocysts; the large and medium p-amastigophores. They were relatively more numerous in some cnidosacs than in the anemones. Large p-amastigophores were often more abundant than large b-mastigophores, the most common acontia nematocysts. Small nematocysts were more abundant in the nudibranch faeces compared to in the cnidosacs. The adhesive spirocysts, which are not useful for self-defence for the nudibranchs, were not found in the cnidosacs or in the cnidophages. Spirocysts were ejected with the faeces. Stored nematocysts might partly mirror which anemone structure the nudibranch had eaten. The larger nematocysts, from the acontia of the anemones, dominated in many cnidophage cells. Expect for in the acontia, medium b-mastigophores were the most common sea anemone nematocysts. In some cnidosacs medium b-mastigophores were the most abundant nematocysts. A. glauca cnidophages had the structure of an elongate bag, with an opening apically, through which nematocysts were ejected. The cnidophages were parallel oriented, with their apical end towards the longitudinal centre of the cnidosacs. In its broad base a large nucleus and cytoplasm were present. The cnidophage narrowed from its broad base towards its apical end. Inside the cnidophages the elongate, mostly closely packed anemone nematocysts were parallel oriented. Their openings pointed all towards the apical cnidophage end with the opening. A. glauca cnidophages were slightly longer than the longest nematocysts of the favorite prey of the nudibranch, the S. viduatus anemone. Large M. senile nematocysts, too long to fit inside the cnidophages of A. glauca, were ejected with the nudibranch faeces. In light squash preparations of A. papillosa cerata, narrow strings of undamaged cnidophages could be pressed out of the cnidosac. A. papillosa cnidophages varied in size and shape, and the number of their stored nematocysts varied from 1- 5 up to 15-20. No opening was visible in the apical cnidophage cell. Basally, the large nucleus and some cytoplasm were visible. Inside the larger, most common cnidophages, nematocysts were closely packed, and their number was not possible to count, as long as the cnidophage was intact. Nematocysts too long to fit inside the smaller cnidophages had only their apical capsule ends inside the apical cnidophage. Their basal capsule ends, outside the cnidophage, were sometimes spread apart in the squash preparations. However, the undischarged nematocyst capsules were still kept together by the apical cnidophage. The smallest cnidophages were of the size of the smallest A. papillosa nematocysts. Often they contained one single or 2-4 small nematocysts. The somewhat larger cnidophages contained 2-4, or more nematocysts of different sizes, the largest capsules with their basal end outside the cnidophage. Empty cnidophage cells, presumably representing different developing stages, were of different sizes and irregular shapes. Some cnidophages were connected to each other. A big nucleus was visible in the cytoplasm. The cytoplasm surrounded the inner cell wall, leaving an inner canal free of substance. In mucus strings, excreted from the nudibranch anus, cnidophage cells filled with nematocysts, small packages of free nematocysts fitting in size into the cnidophages, and strings of closely packed, abundant, free nematocysts were present. Nematocysts in the packages and in the strings were parallel oriented, with their apical capsule end pointing in the same direction. In both A. glauca and A. papillosa cnidae, less useful in self-defence for the nudibranchs, were rejected with the nudibranch faeces. In some faeces discharged or undischarged spirocysts were by far the most abundant cnidae. Small nematocysts were more common in the faeces than larger nematocysts, except for the largest M. senile b-mastigophores in the faeces of A.glauca. Large p-amastigophores were rare or missing. Additional observations and conclusions: Immature nematocysts, early in development, were not found in the cnidosacs. Only a few elongate p-mastigophores, late in development, with the same capsule structure as mature ones were indentified here. Obviously most nematocysts did not complete their maturation in the cnidosacs as proposed by Grennwood and Mariscal (1984). The shape and size of the cnidophages seemed to have an important roll in the transportation of nematocysts. No hydrozoan and scyphozoan nematocysts have been found in the cnidosacs. The rounded to oval size of these nematocysts might inhibit their storage in the cnidophages and cnidosacs. Hydrozoan and scyphozoan nematocysts were found in the nudibranch faeces. The sorting and rejecting mechanisms and the transportation of the nematocysts in the nudibranchs are still unknown but must be regarded as a masterpiece.