Description (from Thorne, 1974)
Bodies practically straight and cylindroid except anteriorly. Lateral fields 1/3 body width. Lip region 1/4 width of neck base, generally set off by a partial view of amphid apertures due to the twisted anterior portion of the body, but probably rounded if a true lateral view could be secured. Tooth very acute, cylindroid-deltoid about 20 um long, the basal portion difficult to determine. Esophagus slender in anterior 1/3 then gradually expanding until posterior 1/2 fills body cavity. Musculature so dense that esophageal gland nuclei not observed. Cardia obscured by large glands. Intestinal cells packed with dark brown granules. Vulva and vagina similar to those of A. aquaticus. Ovaries and uteri unusually long. Uterine spermatheca 4-5 times as long as body width, packed with sperms. Prerectum length 4-5 times body width. Male with the usual thick, slightly arcuate spicula and small furcate lateral guiding pieces. Four elongate, slightly elevated supplements. Diagonal copulatory muscles a prominent feature. Aquatides rotundicaudatus is immediately distinguished by the narrow lip region, short esophagus, very elongated gonads and elongate hemispheroid tails.
Habitat: Small swamp area, Experiment Station field, Brookings, South Dakota.