Pseudodifflugia horrida Penard, 1902

Diagnosis: Cell covered by a greyish or brownish envelope, membranous, flexible, felt‑like, and sprinkled with brilliant amorphous grains and small hyaline scales that penetrate it entirely. These various elements are largely implanted edgewise into the envelope, especially toward the posterior part where they are larger in size, and are often represented by bacillary fragments or diatoms fixed to the surface by their point, giving this region the appearance of an armature. Near the aperture, the envelope is highly plastic and can expand considerably, either to open the buccal orifice widely, or, on the contrary, to close it completely when necessary.

Dimensions: Penard (1902): Length 26–(40–45)–62 µm. Cash, Wailes and Hopkinson (1915): 35-60 µm. Siemensma (Arcella.nl, 2026): up tp 94 µm (Sweden).

Ecology: Freshwater.

Geographical distribution: Type locality small marsh in l’Asile de Veuillards, Switzerland (Penard, 1902); England, Cheshire, Scotland, Loch Ness, Inverness-shire and Ireland, Inishbofin, Galway (Cash, Wailes and Hopkinson, 1915); Czechia (Bartos, 1940, 1954); The Netherlands and Sweden (Siemensma, Arcella.nl, 1982 and 2026 resp.).

Remarks: The combination of a thin flexible envelope, especially around the aperture, and larger xenosomes at the fundus, makes Pseudodifflugia horrida morphologically distinctive among similar species.

A characteristic feature of this species is the constant presence of elongated parasitic rods or filaments, very thin, although more easily visible than those of Pelomyxa, to which they are related. They occur in considerable numbers within the bluish cytoplasm of the anterior part of the body.