Some aspects of the ultrastructure of Gotocotyla secunda and Hexostoma euthynni.

The ultrastructure of the spermatozoon, the protonephridia, the intestine and the tegument of Gotocotyla secunda (Monogenea, Gotocotylidae) and of the intestine of Hexostoma euthynni (Monogenea, Hexostomatidae) is described. The "connecting cells" in the intestine of at least some Monogenea show secretory activity and are possibly involved in extracellular digestion. Sloughing of whole haematin cells appears to be common and may be an important component in the digestive process. Flame cells appear to be a very conservative element and their more or less identical structure in all major groups of parasitic platyhelminths indicates that parasitic platyhelminths have originated from one or a few closely related groups of freeliving Turbellaria with the same type of flame cell.