Phagocytosis and storage of structures or cells from one organism inside the cells of another is fundamental to early eukaryote evolution. However, multiple metazoan lineages have secondarily evolved the ability to sequester structures from their diet. Many groups have evolved such interactions for metabolism (i.e., dinoflagellate symbiosis in corals), but few have done so to boost defensive capabilities. The processes of recognition, phagocytosis, and long-term stability of sequestered cells has been relatively well-studied in cnidarian-dinoflagellate symbiosis. By comparison, little is known about these processes in organisms that sequester putatively defensive structures, such as nematocysts, or how those processes compare with symbiotic interactions. Here, we investigate the underlying mechanism of nematocyst sequestration in an emerging gastropod model, Berghia stephanieae, an aeolid nudibranch well known for its voracious predation on the anemone Exaiptasia. During the digestive process, putatively immature Exiptasia nematocysts move through the digestive tract to a structure called the cnidosac, where they are phagocytosed by cells called cnidophages. We studied features of the cnidophage cell type using traditional histological methods, antibody staining, and differential expression analysis. We identified key morphological differences within the cnidophage cell type, along with 65 upregulated transcripts, including those that may be associated with cnidophage development or with the instigation of phagocytosis. We are also developing methods, such as in situ hybridization, to validate the functional significance of these candidate genes. Since Exaiptasia is already well-studied, the development of such tools in Berghia will provide a unique system in which both sides of the sequestration process can be studied in detail.