We performed short-term cultivation of alive interna fragments of Sacculina polygenea Liitzen & Takahashi, 1997 isolated from the organism of its host, the coastal crab Hemigrapsus sanguineus (De Haan) and studied the histology of parasitic interna and their relationships with the gut and gonads of the host. It is shown that the colonial interna of S. polygenea comprises an extensive trophic system (roots and trunks) and a reproductive system represented by numerous primordia of externae at different stages of development. We observed asexual reproduction (blastogenesis) in the interna beginning with the development of numerous nuclei, dense aggregations of non-differentiated embryonic cells surrounded by a simple epithelium; later on each nucleus develops a primordium of a trophic module. An ovary with paired colleteric glands, paired male receptacles, and mantle with inner cavity and mantle opening arise in the anlagen of externae prior to their emerging to body surface of the crab, i.e., in the interna. The process of oogenesis begins in the primordium of the ovary; in late primordial externae we have found vitellogenic oocytes. Cells performing functions of ingestion, treatment, and accumulation of nutrients are located in the distal parts (rootlets) of the trophic system; the cells filled with nutrients then release the latter into root lumen, where the nutrients are conveyed toward the primordia and, later on, adult externae. Thus, we have visualized the colonial structure of S. polygenea interim comprising numerous elements of reproductive and trophic systems.