Arcella

[ahr-sel-uh]

noun

a genus of freshwater, ameboid protozoa that secrete a hard, umbrellalike shell having a single opening through which the pseudopodia extend.

Origin of Arcella

1838; < New Latin, equivalent to Latin arc(a) a chest, box + -ella diminutive suffix

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Dujardin points out very clearly the identity of structure between organisms like Amœbæ and such forms as Difflugia and Arcella.
The Ocean World:
Louis Figuier