Germinal elements and their development in Echinostoma caproni and Echinostoma paraensei (Trematoda) miracidia

Among the large cells located in the posterior of Echinostoma caproni and E. paraensei miracidia are secretory cells, germinal cells (GC), and undifferentiated cells. Secretory cells do not give rise to progeny, whereas GC do. Undifferentiated cells develop into GC that can also divide to produce embryos. Cleavage of GC of E. caproni occurs only after the parasite has entered the snail host and develops into a sporocyst. With E. paraensei, GC are larger than noted for E. caproni, and in 3 of 23 miracidia examined, germinal cell cleavage had occurred in the miracidium such that an embryo containing 20-25 blastomeres was present. Observations on the germinal elements of miracidia help to explain previous results showing that (1) E. paraensei sporocysts release mother rediae a few days earlier than do sporocysts of E. caproni, and that (2) a single mother redia is produced ahead of all others by sporocysts of E. paraensei, but not by sporocysts of E. caproni. The present study adds support to the concept that E. caproni and E. paraensei are distinct species.