Life Cycle: G. humbargari uses fish as intermediate hosts, and birds as the definitive hosts (Musaffar and Jones 2004).
Body elongated, slightly constricted near the anterior quarter and also near the posterior quarter of its length, 2.032 (1.800-3.015) mm long by 0.348 (0.270-0.465) mm wide; cuticula covered with spines, stouter and more numerous on anterior third of body; oral sucker subterminal, 0.080 (0.064-0.126) mm in diameter; rudimentary ventral sucker preequatorial; prepharynx 0.192 (0.147-0.241) mm long, slender, with enlargement near pharynx; pharynx 0.056 (0.053-0.075) mm in diameter, surrounded with pharyngeal glands; esophagus 0.014 (0.003-0.028) mm long; intestinal caeca wavy or smooth, extending to posterior tenth of body length. Excretory pore terminal; excretory bladder median, within coils of uterus, V-shaped and extending to posterior end of posterior testis; prominent paired ducts from each side of bladder extend along inner margins of caeca to region of genital sinus. External genital opening surrounded by rudimentary sucker; genital sinus 0.119 (0.090-0.162) mm in diameter, occupied by two piriform gonotyles, the protrusible portions of which are armed with several rows of spines; internal genital opening on left side of genital sinus; seminal vesicle large, divided by one or two constrictions into an anterior S-shaped chamber surrounded by many prostate cells and a posterior globular chamber immediately anterior to ovary. Testes globular, postequatorial, obliquely one behind the other: anterior testis 0.117 (0.075-0.144) mm in diameter, located more or less toward the left of body; posterior testis 0.109 (0.068-0.162) mm in diameter, located more or less toward the right of body. Ovary globular, 0.097 (0.079-0.165) mm in diameter, directly behind seminal vesicle; seminal receptacle directly behind and slightly to right of ovary, 0.127 (0.079-0.187) mm in diameter; oviduct emerges from mid dorsal surface of ovary; Laurer's canal present; diffuse shell gland immediately posterior to ovary; vitellaria sparsely developed, intercaecal, extending from the posterior margin of ovary to near posterior end of body; uterus fills intercaecal area, not otherwise occupied, from posterior margins of seminal vesicle to posterior end of body; eggs yellowish-brown, numerous, 0.0228 x 0.0144 mm. The above measurements were obtained from twenty specimens.
Sources: Park, J.T. 1936. New Trematodes from Birds, Plagiorchis noblei sp. nov. (Plagiorchidae) and Galactosomum humbargari sp. nov. (Heterophyidae). Transactions of the American Microscopical Society, Vol.55, No.3, p. 360-36.
Muzaffar, S.B. and Jones, I.L. 2004. Parasites and disease of the Auks (Alcidae) of the world and their ecology- a review. Marine Ornithology, Vol. 32, p. 121-146.