Life Cycle: Helisoma snails act as the first intermediate host, fishes as the second host, and the definitive host is a bird of some kind, often herons (Hunter, 1934).
Body oval, elongated, with truncated forebody, wider in gonadal region, 5.9–8.2 (7.0 ± 0.791) mm long, 1.3–2.8 (2 ± 0.387) mm wide. Oral sucker small, 171–394 (311 ± 58) long × 252–501 (399 ± 67) wide, surrounded by a well-developed oral collar. Pharynx not observed; esophagus very short, bifurcates immediately posterior to level of oral sucker, esophageal bulb present. Ventral sucker larger than oral, 601–918 (764 ± 106) long × 583–966 (756 ± 120) wide. Intestinal ceca run to posterior end of body, with slightly indented margins both in pre- and postacetabular regions. Excretory ducts in complex web draining into excretory system, not clearly visible. Testes between middle and posterior third of body; anterior testis, 290–743 (519 ± 149) long × 569–1,141 (802 ± 190) wide, triangular, slightly lobed, compressed laterally by cirrus sac and dorsoventrally by uterus, when filled with eggs; posterior testis, 166–587 (461 ± 115) long × 379–1,414 (790 ± 259) wide, triangular and median, stretching across intracecal space. In some specimens efferent ducts were visible, originating from middle part of posterior margin of anterior testis and from middle part of anterior margin of posterior testis, both ducts reaching posterior end of cirrus sac. Cirrus sac, 151–795 (327 ± 183) long × 166–741 (497 ± 212) wide, anterior to ovary, surrounding laterodorsally right margin of anterior testis and displacing its anterior lobe to left. Genital pore anterior to anterior testis, not clearly visible in most specimens. Ovary small, 118–306 (175 ± 61) long × 101–267 (186 ± 53) wide, ovoid or rounded in shape, in some specimens slightly lobed, located in intertesticular space on right side of body. Vitellaria extensive, extending from posterior margin of ventral sucker to almost reaching end of ceca. Uteroduct runs around left margin of anterior testis and opens into uterine sac, which occupies almost all space between ventral sucker and anterior testis when filled with eggs. Metraterm muscular, connecting uterus to genital atrium. Mature eggs, 101–109 (105 ± 3) long × 63–79 (69 ± 5.8) wide, in uterine sac, some in uteroduct and in ootype complex space.
Metacercariae: Similar to adult, but slightly smaller. Oral collar well developed. Ventral sucker larger than oral. Forebody with glandular structure, anterior to ventral sucker. Intestinal ceca lateral to ventral sucker and genital primordia, characterized by visible intestinal pouches especially in postacetabular part. Testes less conspicuous and with greater digitation than in adults. Cirrus sac well developed, crescent shaped, compressed against left anterior margin of anterior testis. Genital pore opening lateral to right anterior margin of anterior testis. Ovary irregular in shape, smaller than cirrus sac, located dextrally in intertesticular space. Vitellaria not evident. Tubular proximal uterus follows similar pathway to that in adult; muscular metraterm visible. Excretory ducts connecting at posterior extremity just posterior to cecal ends. Cuticular surface covered by thin spines.
Sources: Caffara, M., Locke, S.A., Marcogliese, D.J., and Fioravanti, M.L. 2011. Morphological and molecular differentiation of Clinostomum complanatum and Clinostomum marginatum (Digenea: Clinostomidae) metacercariae and adults. Journal of Parasitology, Vol. 97, Issue 5, p. 884-891.
Hunter, G.W. 1934. The life cycle of the yellow grub of fish, Clinostomum marginatum (Rud.). Journal of Parasitology Urbana, Vol. 20, p. 325.