Mature acanthors consist of three syncytia, the central syncytium (median), the epidermal syncytium (caudal), and the frontal syncytium (Fig. 1). Within the central syncytium, there are ten subepidermal longitudinal muscles and two more centrally located retractor muscles. According to descriptions of Albrecht et al. from acanthors (of three species) that were still inside the mother’s body cavity and enclosed by eggshell envelopes, the subepidermal muscles are connected via cytoplasmic bridges with the central nuclear mass. However, in hatched acanthors of Paratenuisentis ambiguus collected from the gut of its crustacean intermediate host, no connections between the central syncytium and the subepidermal muscles could be found; the muscles were part of the epidermal syncytium (Fig. 1). So probably the bridges described may get lost during the final maturation of the acanthor or the hatching process. The central nuclear mass contains condensed as well as decondensed nuclei. The latter are also found in the epidermal and the frontal syncytium (Fig. 1). The epidermal syncytium forms the outer surface and most of the larva’s body. It contains numerous vacuoles with mucus-like electron-lucent content which are concentrated near the surface of the anterior half (Fig. 1). In acanthors of P. ambiguus , the crypts of the outer membrane are fused underneath the larva’s surface and harbor electron-dense granules which may have a function during the penetration of the larva into the hemocoel of the intermediate host. The electron-dense vesicles as well as the vacuoles with electron-lucent content inside the frontal syncytium could probably also be involved in the task of penetration, chemically supporting the action of the hooks. Stimulated acanthors of Moniliformis moniliformis have been found to discharge chitinase, but this enzymatic activity has not been localized at the acanthor’s body. Hooks are most prominent at the anterior surface and decline in size toward the larva’s posterior end (Fig. 1). Acanthors are rich in glycogen in the cytoplasm between the muscles, nuclei, mitochondria, and inclusions (Fig. 3a).

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Mature acanthors of acanthocephalans are enclosed by four eggshells separated by interstices containing granular electron-lucent material (Figs. 2, 3, and 4a). However, eggs of Neoechinorhynchus species become complemented by a fifth envelope (E0) creating a fifth voluminous outer interstice (Fig. 2c). The outermost, first envelope seems to derive from the “fertilization membrane.” Usually, it is thin but can be reinforced by outgrowths of the underlying eggshell (Figs. 3b and 4a). This envelope (E2) was found to contain keratin in all three groups of the Acanthocephala. In palaeacanthocephalans, it forms more or less filiform outgrowths (Figs. 2a and 3a, b) entangling with algae or leaves (the food substrates of the intermediate hosts) once the outermost envelope has disintegrated in the water. In archiacanthocephalans, the second eggshell is interspersed with the respective outermost interstice and seems to function in protecting the egg from desiccation and other negative outer influences (Figs. 2b and 4a). Among archiacanthocephalans, the underlying tripartite third envelope also comprises keratin, while eoacanthocephalans and palaeacanthocephalans do not have keratin in this eggshell. The fourth, innermost eggshell contains chitin among palaeacanthocephalans and archiacanthocephalans (Fig. 3c). In eoacanthocephalans, however, this innermost eggshell lacks chitin. The interstices contain carbohydrates which together with the envelopes seem to have different functions.

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