Arcella Visual Key – Microworld

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Aperture usually surrounded by pores. Aperture diameter 0.45 of shell diameter. Shell diameter 70-160 µm.

Arcella discoides

Aperture surrounded by pores. Aperture diameter ≥ 0.4 of shell diameter. Shell very flat; diameter 140-402 µm.

Arcella megastoma

Aperture surrounded by pores. Distinct basal rim. Height ca 1/3 of shell diameter.

Arcella artocrea

Arcella arenaria

Shell elliptical, quadrate or circular. Shell often with distinct dorsal folds or ribs. Aperture small and with pores.

Arcella catinus

Shell with six or more ribs; dorsal surface with ribs and depressions; aperture smooth, without crenulations.

Arcella conica

Shell with flattened top and aperture with crenulated border; usually with six ribs.

Arcella n.sp.

Shell in side view trapezoid, with six or more ribs; aperture smooth without crenulations.

Arcella costata

Shell in lateral view trapezoid, with 5-7 ribs. Aperture with pores.

Arcella jurassica

Shell with basal border. Dorsal surface with depressions. Diameter c. 80-110 µm.

Arcella gibbosa

Shell with basal border. Dorsal surface with depressions. Diameter c. 80 µm.

Arcella intermedia

Shell in lateral view hemispherical, with acute or somewhat rounded corners, sometimes with a basal border. Diameter c. 35-55 µm.

Arcella hemisphaerica

Shell hemispherical, with smooth and broad rounded corners in lateral view.

Arcella rotunda

Shell with distinct basal border. Ratio diameter aperture/shell 0.2.

Arcella vulgaris

Shell dome shaped; aperture with irregular crenulated border.

Arcella crenulata

Shell formed like a saddle. Aperture elliptical.

Arcella excavata

Shell balloon shaped.

Arcella mitrata

Test broader than high; height c. 80-90 µm.

Arcella spectabilis

Shell with undulations, more or less spherical.

Arcella peruviana

Shell spherical, wrinkled.

Arcella nordestina

Shell shaped like an old hat.

Arcella rukiensis

Shell resembling a pie; prominent folds or ribs often with darker color.

Arcella formosa

Shell with striated border

Arcella brasiliensis

Shell with a number of projections.

Arcella dentata

Shell border with numerous small spines.

Arcella rota

Shell flattened, with crenulated aperture.

Arcella multilobata

Shell crowned with some spines.

Arcella corona

Shell with high dome and distinct neck.

Arcella apicata

Shell like Gandalf’s hat.

Arcella gandalfi

Shell sub-hemispherical, round smooth aperture, thickened edges

Arcella jeanneli

Ferry Siemensma
Created March 4, 2019
Last updated February 13, 2021

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