Urceolus sabulosus from the river's edge sapropelic benthos of Peconic River. Length is 50 um. Though described as a freshwater species, I have repeatedly found an identical organism in the local estuarine bay.

From the original description by Stokes 1877: https://www.jstor.org/stable/pdf/983073.pdf
Urceolopsis (Urceolus) gen. nov.
Animalcules free-swimming, flask-shaped, soft, flexible and elastic, the entire cuticular surface more or less covered by adherent, irregular and angular sand grains; otherwise essentially as in Urceolus.
Urceoopi scabuIosa, sp. nov. Fig. 3.
Urceolus sabulosus, Stokes; Am. Monthly Micr. Journ., vii, May, 1886.

Body flask-shaped, soft, flexible, elastic, normally compressed and somewhat gibbous, about twice as long as broad, widest centrally, obtusely pointed posteriorly, the entire surface more or less covered, often almost concealed, by adherent, irregular and angular sand grains; anterior extremity constricted to form a short neck-like prolongation, the circular border thickened, expanded, and obliquely truncate; flagellum large,
equaling or exceeding the body in length; nucleus not observed; con-
tractile vesicle (?) single, laterally placed near the anterior extremity;
pharynx apparently extending to near the body-centre. Length of body 1/500 inch. (50 um) Habitat Fresh water with Algae.

Euglenophyta: Heteronematales: Astasiidae Urceolus sabulosus
Stokes 1877
Family: Similar to Euglenidae in body form and general structure, but without chromatophores (chloroplasts); body highly plastic, although usually elongate spindle.

Genus: Body colorless; plastic; flask-shaped; striated; a funnel-like neck; posterior region stout; a single flagellum protruded from funnel and reaches inward the posterior third of body (Kudo, 1966).
Species: Spindle-form; covered with minute sand-grains; about 58 μm long; fresh water (Kudo, 1966). 50-60 μm long, with attached sand grains.