Specialized morphology of diverse parasitic crustaceans reflects their adaptations to an endoparasitic lifestyle. Rhizocephalan barnacles are one of the most highly modified obligatory parasites of other crustaceans. Comprehension of the functional morphology of rhizocephalans could elucidate the main evolutionary trends not only inside parasitic barnacles, but in parasitism as a whole. Despite that, the available morphological information on the rhizocephalans is very fragmented. In this study, we examined the organization and ultrastructural features in different parts of the interna of Peltogaster reticulata (fam. Peltogastridae). The main trunk cuticle is much thicker than that of the side branches due to the different functions of these body parts. The central lumen in the main trunk is lined by an extracellular matrix, while the side branches are not. Muscular fibers are only present in the body wall of the main trunk, where they are organized as a “wicker basket”. Furthermore, functional differentiation can be found at the ultrastructural level in the cells of the rootlets: there are distinct cell types both in hypodermal and axial cell layers. The rootlets of P. reticulata are covered by a network of the host's neurons and capillaries.