world of amoeboid organisms

Arcella megastoma
Large shell, 370 µm in diameter

Arcella megastoma  Penard, 1902

Diagnosis: shell very flattened with a wide aperture; ratio diameter aperture/shell usually 0.45-0.53; with 2 to 200 nuclei.

Dimensions: diameters of the tests 140-402 µm; diameter of aperture 40-216 µm. My measurements 197-370 µm, diameter of aperture 87-190 µm.

Ecology: marshes and ponds, among algae. Common, but never abundant.

Remarks:I have measured the diameter of the aperture and the shell of a number of discoid Arcella specimens from several locations in the Netherlands. The results are plotted in the graph below and shows the existence of at least two different groups, probably two species, Arcella discoides and Arcella megastoma.

Arcella megastoma
Arcella megastoma
Arcella megastoma, aperture with pores.
Arcella megastoma
Arcella megastoma in lateral view
Recent posts

Penardochlamys arcelloides

P. arcelloides – from Penard, 1904 Penardochlamys arcelloides  (Penard, 1904) Deflandre, 1953 Diagnosis: Colorless or very slightly yellowish envelope, bag or cauldron-shaped, very thin, chitinous,

Read More »

Reticulomyxa hannae

R. hannae, plasmodium – photomicrograph Steffen Clauss Reticulomyxa hannae Völcker and Clauß, 2020 Diagnosis: Trophozoites surrounded by a very thin mucous envelope, with slowly moving plasmodia, variable

Read More »

Velamentofex saxonensis

V. saxonensis, feeding (photomicrograph Steffen Clauss) Velamentofex saxonensis Völcker and Clauß, 2020 Diagnosis: Trophozoites ovoid, 30-95 μm, granuloreticulopodia up to 1000 μm long. with a hyaline, thin

Read More »

Velamentofex tyrolensis

V. tyrolensis (photomicrograph Steffen Clauss) Velamentofex tyrolensis Völcker and Clauß, 2020 Diagnosis: Trophozoites ovoid, 98-125 μm, with reticulopodia up to 300 μm long, with a hyaline,

Read More »

Velamentofex berolinensis

V. berolinensis Velamentofex berolinensis Völcker and Clauß, 2020 Diagnosis: Trophozoites ovoid, 50-300 µm, with a hyaline, extremely flexible, membranous test. Nuclei numerous, 11.6-16.2 µm in diameter,

Read More »


 Lieberkuehniidae  Siemensma, Holzmann, Apothéloz-Perret-Gentil, Clauß, Voelcker, Bettighofer, Khanipour Roshan, Walden, Dumack and Pawlowski, 2020 Diagnosis: Multinucleate organic-walled monothalamous foraminifera. Test very flexible, elongated to broadly

Read More »