Triaenophorus coracidium

The coracidium emerges from the egg a few days after the eggs is deposited into freshwater. The dark cellular mass at the centre is the oncosphere, which will penetrate the gut of a copepod and develop into a procercoid. The thick, clear layer with scattered nuclei is a layer that inflates after the egg hatches, and is coated with cilia (although the cilia are not visible in this photo). The coracidium can swim for a few days, which both maintains its position in the water column where its first intermediate host copepods frequent, but the motion also may act to attract predation on it by the copepods, enhancing its probability of transmission.