austinjosephhorenkamp's Identifications

The water sample taken (1 liter) had many of these oligochaetes in it.

A water sample was taken from the shore of lake Istok. The air temperature was 24°C (75.2 °F). The sample was stored at room temperature and observed 2 days after collection.

Video: https://youtu.be/Mq1ohPvAlOE

Stylaria fossularis from the northernmost edge benthos of the freshwater coastal pond at Ocean Dunes Apartments in the Atlantic Double Dunes Reserve. The worm measures approximately 3 mm in length. There are no eyes and the proboscis starts from the end of the conical prostomium rather than from invaginated lobes which is usually a feature of S. lacustris.

From:
https://www.nies.go.jp/chiiki1/protoz/morpho/oligocha/o-stylar.htm#Stylaria%20fossularis

Stylaria fossularis Leidy, 1852 (ref. ID; 1923, 3692, 7854)
Descriptions
Proboscis projects from the apex of the prostomium. (ref. ID; 1923)
Remarks
Respecting the authority of Michaelsen (1900), several authors have regarded S. fossularis as an intraspecific form of S. lacustris. Sperber (1948) listed characters different in these species. I did not find any difference in size between them, noted by Sperber (1948), as my S. fossularis had ventral chaetae about 100-150 µm long and hair chaetae 140-820 µm, which was well within the variation limits of S. lacustris. The shape of the prostmium was certainly different, conical in S. fossularis and bilobate in S. lacustris, without any intermediate forms. (Di Persia 1975 described variations of the prostomium in S. lacustris, from lobate to rectangular, but never conical like in S. fossularis. According to my own experience, a rectangular prostomium occurs in the early separated zooids of S. lacustris). Ventral crotchets of S. fossularis, although curved in thier proximal portion (well figured by Liang 1962), had no broken appearance characteristic of S. lacustris. The atrium of the mature individual was distinctly divided into the ampulla and the ejaculatory duct (the duct being inconspicuous after Sperber 1948), both about 90 µm long. Vas deferens entered the atrium between the ampulla and the duct, outside the atrial prostatic layer, as noted by Sperber (1948) in the case of S. fossularis. Thus S. fossularis and S. lacustris appear to be two different sympatric taxa in the study area. (ref. ID; 7854)

The following discussion was taken from: https://greatlakescenter.buffalostate.edu/stylaria-lacustris
Stylaria lacustris (Linnaeus, 1767) is a freshwater oligochaete worm that is uncommon in the Great Lakes. It is a naidid worm that has dorsal bundles with hair starting in VI, with or without eyes, and usually having a proboscis, although it may be broken. Dorsal bundles contain 1–3 hair chaetae and 3–4 simple-pointed needles without a nodulus. There are 4–14 ventral chaetae per bundle with the upper tooth longer than the lower but more importantly a double bend at the proximal end. The long proboscis usually starts from between two invaginated lobes, although it may start directly from the end of the prostomium. When it has invaginated lobes, the head may be somewhat flattened dorsally, and if eyes are present, they will be along the lateral margin below the invaginated lobes. S. fossularis may be a synonym, but it is accepted as a separate species by ITIS. S. fossularis was originally separated by lacking the invaginated lobes and having the proboscis starting from the end of the prostomium, as well as some specimens lacking the extra bend in the ventral chaetae. Kathman and Brinkhurst considered it a synonym of S. lacustris because there may be plasticity in the proboscis characteristics of S. lacustris and it was shown that it could change in culture (Kathman and Brinkhurst, 1998).

Distinguishing features:
S. lacustris is identified by the dorsal bundles with hair chaetae in VI, ventral chaetae with a double bend near the proximal end, and proboscis usually with invaginated lobes. The dorsal chaetae are simple-pointed, and the ventral chaetae have the upper tooth longer than the lower. There may or may not be eyes. This species usually has a proboscis but in practice the proboscis may be torn off or obscured when mounted. In that case, rely on the chaetae. Zooids that have detached may be identified by the ventral chaetae even if they are lacking proboscis and have a flat, rectangular prostomium.

Imaged in Nomarski DIC on Olympus BH2 using SPlan 10x and 20x objectives plus variable phone cropping on Samsung Galaxy S9+.