Nudibranch mollusks (Gastropoda: Heterobranchia) are widely known for their ability to incorporate some active biochemical compounds of their prey, or even organelles and symbionts of the prey, which assured biological success of this group. At the same time, the process of nematocysts obtaining and incorporation into specific structures called cnidosacs by cladobranch mollusks remain poorly studied. This highlights a necessity of additional ultrastructural studies of cnidosac and adjacent organs in various aeolid mollusks using modern microscopic methods as they may provide new insight into the cnidosac diversity and fine-scale dynamics of nematocysts sequestration process. The present study is focused on the general and fine structure of the cnidosac area in cladobranch Aeolidia papillosa (Aeolidiidae). Specific goals of our study were to provide a detailed description of histological and ultrafine structure of epidermis, upper parts of the digestive glands and the cnidosac, its innervation and proliferation using standard histological techniques, confocal laser scanning microscopy (CLSM) and transmission electron microscopy. Our results clearly demonstrated that A. papillosa cnidosac is a much more complex structure, than it was thought, especially compared with simple cnidosacs found in flabellinids and facelinids. Using CLSM for functional morphological analysis provides a better resolution in visualization of structural elements within a cnidosac compared with traditional histological techniques. We revealed the presence of two cell types in the cnidophage zone: cnidophages and interstitial cells, which differ in ultrastructure and function. Our results also document the presence of a specific cnidopore zone, lined with differentiated cuboid epithelium bearing long microvilli, which likely provides a unidirectional flow of nematocysts during kleptocnides extrusion. For the first time, occurrence of vacuoles containing protective chitinous spindles in the cnidosac epithelium was shown.