Ultrastructure of metacercarial cysts of six heterophyid trematodes from fish.

The ultrastructure of metacercarial cysts of Ascocotyle sexidigita. A. (Phagicola) diminuta, and Pygidiopsoides spindalis (herein described for the first time) collected from natural hosts in California was compared with that of cysts from A. tenuicollis (also described for the first time), A. pachycystis, and A. leighi collected from natural fish hosts in Mississippi. The results show that none of the cysts studied approximated the thick cyst produced by A. pachycystis, but all cysts comprised several layers of putative glycoprotein except for A. (P.) diminuta, which was associated with a single layer of spongy cyst wall.