miracidium of Megalodiscus temperatus from Rana pipiens.
This is a section through the coils of the excretory duct leading from the flame
cell. The large body in the center is the nucleus of the cells which
develop into the ducts. There are a number of nuclei along the length of
the duct though not seen here. Note that the ducts have indentations in the surface, increasing
internal surface area, and suggesting there may be a reabsorptive function. The
black granules throughout the frame are glycogen granules. This miracidium
is non-feeding and uses glycogen as its principle energy source while seeking
its snail host. 20,000x.