Description: "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" (Park, 1936).
Life Cycle: G. humbargari uses fish as intermediate hosts, and birds as the definitive hosts (Musaffar and Jones 2004).
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.