Life Cycle: The first intermediate host are Helisoma snails, the second intermediate host are ostracods, a possible paratenic host are nymphs, and the definitive hosts are amphibians (Macy, 1960).
Present specimens, 37 in number, vary in length from 1.61 to 6 and from 0.43 to 1.5 in greatest breadth. A few present pictures approximating to that given by Looss (Die Dist. etc. 1894. Taf. III. fig. 49) but they are shorter and more compact in proportion to their breadth, being broad and deep at the centre and gradually narrowing towards the ends which are rounded. Sections are nearly circular. The cuticle is smooth and perforated by numerous skin-glands. The ventral sucker is larger than the mouth-sucker and in the middle of the ventral surface. The mouth passes into a pharynx which is succeeded by a short oesophagus and then the lateral coeca, broad and wavy, extending to the extreme posterior end. The excretory bladder passes from the terminal pore to the ventral side and divides, just behind the ventral sucker, into ventro-lateral vessels which run forwards and unite above the mouth-sucker, just anterior to the transverse band of the nervous system. Most of the posterior half is taken up by the genital glands — the anterior testis being often close behind and to one side of the ventral sucker while the vitellaria are in the extreme posterior end. In all the testes are oblique (not side by side) and in three quarters of them the anterior testis is on the right side, the posterior on the left. In half the ovary is in the median line; when slightly displaced it is on the same side as the posterior (nearest) testis, generally left. The vitellaria have five follicles on the left, four on the right — in only two specimens out of eighteen were the five on the right and then the ovary and posterior testis were also slightly towards the right. When the ovary is in the centre the shell-gland is behind it; when the former is to one side the latter is behind and towards the other side. The Laurer's canal runs upwards and backwards to the surface. A very noticeable difference from the European species is the short space between acetabulum and first testis, limiting the number of transverse folds of the uterus in this region. The genital pore is immediately behind the mouth sucker, underneath the pharynx. There is a short sinus which receives the small vagina and ductus ejaculatorius, the latter succeeded by a vesicula seminalis. The eggs are yellow, have a big rounded end, and then slowly taper towards the little end, which rounds down to the base of the filament. The latter appears shorter than the egg but towards the end it becomes very thin and bent or twisted so that it is hard to get a straight measurement. Eggs generally about 0.063 x 0.018 (sometimes one as large as 0.071 x 0.021), filament about 0.056.
Sources: Macy, R.W., Cook, W.A., and DeMott, W.R. 1960. Studies on the life cycle of Halipegus occidualis Stafford, 1905 (Trematoda: Hemiuridae). Northwest Sciences. Vol. 34, No. 1, p. 1-17.