The cystacanth stage of acanthocephalans is the final stage within the intermediate host. Morphological development is complete. The parasite (stained dark red) has a thick body wall with a triangular-shaped hemocoele. The proboscis is fully developed and retracted, and is visible in this photo as the curved structure in the hemocoele. At the anterior end (top of photo) is a small passage through the body wall where the proboscis will evert to attach to the gut of its definitive host. The cystacanth is surrounded by a clear cyst wall which provides the parasite some protection against defense reactions of its intermediate host. Cystacanths are usually 1-2 mm long and, in life, may be naturally colored from white to orange to red.