Penardochlamys arcelloides  (Penard, 1904) Deflandre, 1953

Diagnosis: Colorless or very slightly yellowish envelope, bag or cauldron-shaped, very thin, chitinous, deformable, folded in the mouth, covered over its entire surface with fine punctuation, but without cellular patterns. Grayish plasma, rounded, containing two large nuclei with central nucleolus, and one or more contractile vesicles. Large, few pseudopods.

Dimensions: Average size 60 µm.

Ecology: Freshwater.

Remarks: (from Penard, 1904) P. arcelloides has a chitinous, hyaline or very slightly yellowish, thin covering, covered over its entire surface with fine punctuations, not very distinct, which on the membrane, seen in cross-section, granulations rather than transverse streaks; but these punctuations have no analogy with the alveolar structure found in the genus Arcella. The test or envelope has the appearance sometimes of a flattened sphere, sometimes of a cauldron, but it is deformable, traversed by more or less pronounced undulations, scalloped with hollows and reliefs, which can slowly change shape and place; it gets depressed, or goes up, and the way it looks varies a little bit from one moment to the next. At its anterior part, the thinner membrane or test wall folds back inside, leaving a rounded-sinuous pseudostome free by which the pseudopods are extended. This pseudostome is rarely visible, both because the membrane has become completely colorless in this region and because it squeezes against the pseudopodia, with which it merges; when the animal is completely withdrawn in its envelope, the pseudostome disappears in plain sight, its edges joining, with folds, to completely obliterate the opening.
The naked body is roughly that of an Arcella; it is a more or less rounded, greyish mass, filled with refractive grains of very variable volume, and separated from the internal wall of the envelope by a fairly wide space. As with Arcella’s too, the plasma has two nuclei, but, unlike what is seen in the latter genus, these nuclei are not normally opposite one another on each side of the pseudostome; they are most often seen next to each other, in the posterior region of the plasma, and without each occupying a well-defined position. Very clear, they show a large greenish-blue nucleolus separated from the nuclear membrane by a liquid margin.

There is a beautiful and large contractile vesicle, which, as a rule, is seen very close to the nuclei; often another appears in the vicinity of the pseudostome; rarely there are more than two.

The pseudopodia are widely developed, similar to those of the Difflugia in general, and remarkably large; there is usually only one left, especially strong and elongated. These pseudopods contain small grains, coming from the endoplasma and running until near their extremity.

The size is on average, in this species, from 60 to 70 measured in the greatest width of the envelope; the height, from the pseudostome to the bottom of the envelope, is very different from one moment to the next, and most often varies between 40 and 50 µm.

This species was only collected by Penard in a small pond near Geneva, December 1902; individuals were rare, but he was able to examine a dozen. In 1903, he could not find this organism anywhere, not even in the locality where it was originally collected.

This species has a certain analogy with Arcella; but the latter, even all very young and clear, have already taken on their final contours, and their rigid envelope, with already clear and regular alveoles, is pierced with a pseudostome that is always round and non-deformable. The resemblance is even stronger with the genus Corycia, which would hardly be distinguished from it except by its envelope in the shape of a bag largely open at its base. But as the genera Corycia and Pseudochlamys are so close together that very young animals do not differ, so to speak, from one genus to another, and that the Corycia’s, special to mosses, assume an adult configuration very different from that of P. arcelloides, it is close to Pseudochlamys patella that Penard thought he should put this new species.