This study presents the structure of the capsule around the acanthocephalan Sphaerirostris picae (Rudolphi, 1819) Golvan, 1956, in its natural paratenic host, Lacerta agilis Linnaeus, 1758. The capsule was composed of 2 layers: a thin, dense inner layer and a loose, thick outer layer. The inner layer was formed by macrophages and multinucleated cells. The outer layer consisted mainly of flattened fibroblasts, which included a small number of macrophages, granulocytes, plasma cells, and pigment cells; the extensive intercellular spaces between these cells were filled with collagen fibers. The acanthocephalans are thought to have particular defense mechanisms that diminish phagocytic activity of immune system cells in the host.

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