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Difflugia subaequalis
Difflugia subaequalis, from Penard, 1910

Difflugia subaequalis  Penard, 1910

Diagnosis: Test almost spherical, slightly elongated, with a wide pseudostome, almost as wide as the test itself. Aperture very slightly eccentric, surrounded by a more or less thickened rim.

Dimensions: Length 80-82 µm, width 88 µm in general.

Ecology: Freshwater. Switzerland.

The original description of Penard, 1910, translated from French:

The shell is clear and almost transparent when young, then a brown which over time becomes very dark. The shell is rigid and roughly spherical in shape. More particularly, one could compare it to a sphere very slightly stretched, then cut abruptly by a very wide truncation, so wide even that the diameter of the opening thus exposed remains only a little less than that of the shell itself, in the most swollen region of the latter. This truncation, or pseudostome, is not geometrically perpendicular to the sagittal axis of the shell, but rather conducted along a plane that may be called eccentric; but this eccentricity, normal moreover, is so weak that it is hardly noticed, and that only when the shell is favorably oriented, by what we would call the side, if we could speak of side face in an object whose cross section is absolutely circular.
The pseudostome is surrounded by a kind of bead, or little pronounced but very clear relief, due to a special accumulation of particles which form the envelope. In this region of the pseudostome, too, the shell appears to be somewhat glutinous, and the animal, when attached to the substratum in the manner of a limpet, clings to it more strongly than one would be entitled to imagine.
This whole shell is made of extremely small, shapeless, siliceous particles or granulations, which under low magnification remain invisible one by one and simply give the shell an ashy appearance. These particles, colorless in themselves, are welded to each other by a chitinous cement, which at first clear and scarce, becomes darker and thicker over time, and is the cause of the brown color that ends up coating the shell.
The plasma fills most of the envelope. We see a contractile vesicle, then behind it a large nucleus, all stuffed with extremely small grains, which represent the nucleoli. Pseudopodia are normal, rather broad and short; the animal seems to deploy them only rarely.
In almost all of the individuals examined, the shell length was found to be remarkably constant, ranging from 80 to 82 µm. As to the width, it is 88 μm in general, that is to say, greater than the length, and this fact is understandable if one thinks of the vast oral truncation, which decreases in a noticeable proportion the length of the longitudinal axis of this pseudo-sphere represented by the shell. In some individuals, however, the shell is longer than it is wide, but this is very exceptional.
This species appeared, in great abundance, in the spring of 1909, in a small pond near Geneva, near the St-Georges firing range.
At first, Difflugia subaequalis seemed to me to be comparable to D. olliformis Lagerheim. It is very close to it, to be sure; but the description of the Swedish professor, a little vague and very laconic, is far from allowing a certain identification; and it would rather introduce an element of confusion into the subject than to unite under one specific name two organisms which may well be quite distant from each other.

Ferry Siemensma, created June 21, 2019; last modified December 11, 2021
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