Description (from Raski, 1975)
Female: Body curved ventrally in an open 'C' shape when in fixative.  Head not set off, tapering with sloping sides to truncate anterior end: tiny lips protrude slightly to give rounded corners to truncate surface.  Stylet slender with backward-directed knobs.  Dorsal gland orifice about 3 um posterior to stylet knobs.  Excretory pore at same level as hemizonid at anterior end of posterior bulb.  Esophago-intestinal valve rounded.  Ovary outstretched, spermatheca large, rounded, filled with spermatozoa.  Vagina with posterior rounded protuberance resembling a small post-uterine branch.  Vulvar flap prominent, rounded.  Tail conoid, narrows gradually, then more abruptly near terminus to give a slight digitate appearance with a rounded, almost acute, tip.  Body annules average about 1 um wide.  Lateral field with three lines, the inner one very faint, outer two more prominent.
Male: Body slightly ventrally curved.  Head not set off, narrows with sloping sides to rounded margins and truncate anterior surface, sclerotization very light.  Stylet absent, esophagus degenerate.  Excretory pore as same level as hemizonid.  Testis outstretched, spicules only slightly curved: anal sheath protrudes distinctly with slight projection at posterior edge.  Tail narrows abruptly posterior to cloaca, curves almost 90 degrees ventrad, then narrows again abruptly near tip (as in the female) to a very fine, rounded, almost acute, terminus.  Body annules average about 1 um wide.  Lateral field with three lines, as in female.

-Raski  notes that P. humilis is most closely related to P. aquaticus from which it differs in its smaller size [.34 mm vs. .18 mm].  Ebsary, 1991 notes that P. humilis is a synonym to P. aquaticus.