Centropyxis aculeata  (Ehrenberg, 1838) Stein, 1859
Basionym: Arcella aculeata Ehrenberg, 1838

Diagnosis: Shell cap-shaped, yellow or brown, ovoid or round and usually with four or more lateral spines; shell built of organic cement, usually more or less densely embedded with quartz grains and diatom frustules, except for a more or less smooth ventral region around the aperture; the aperture is sub-central, invaginated, often with two struts connecting the apertural rim to the dorsal region. Posterior part of the apertural rim often visible as a flat belt-shaped lip.

Dimensions: According to Ehrenberg (1838): up to 111 µm (without spines). Other authors: 92-178 µm in diameter (It is not always clear whether measurements refer to the scale without or including spines).

Habitat: Freshwater; in Sphagnum, other submerged mosses, in sediment of ditches and lakes.

Geographical distribution: Cosmopolitan.

Varieties:
Centropyxis aculeata var. grandis Deflandre, 1929 (without spines 150-180 µm)
Centropyxis aculeata var. gigantea Decloitre, 1965 (without spines >300 µm)

Remarks: Specimens cultured in the absence of extraneous particles produce a shell that is proteinaceous and similar in structure to species of Arcella. When encysted, the body assumes a spherical form in the wider part of the shell; cytoplasm  granulated, colorless or with green globules. Cash (1905) observed that specimens appear to differ widely from the type description and also from each other in size and structure. Deflandre (1929) in his review of the genus described three varieties of C. aculeata which differ in size, shape and the number of spines. In clonal cultures both large and many spined specimens are produced, but these forms represent only a small percentage of the population.
C. aculeata is the type species of genus Centropyxis by subsequent monotypy by Stein, 1859: “…und eine neue, auf Arcella aculeata Ehbg. zu gründende Gattung Centropyxis St.” [Translated from German: “… and, based on Arcella aculeata Ehbg., a new genus Centropyxis to be created.”]

The first mention of Centropyxis aculeata is by Ehrenberg in 1830 as Arcella aculeata, in Abhandl. der Akademie d. Wissensch. zu Berlin, 1830. p. 40. 1831. p. 91. In 1838 he gives following description of this species (Ehrenberg, C.G., 1838 : Die Infusionsthierchen als volkommene Organismen. Leipzig, p. 133-134):

Arcella aculeata, stachliges Kapselthierchen. Tafel IX. Fig. VI.
A. hemisphaerica, saepe difformis, margine aculeata, lorica e fibris bacillaribus brevibus (paleaceis) constante, flavicans.
Arcelle epineuse, hemispherique , souvent difforme, epineuse au bord, ayant la carapace formée de fibres bacillaires courtes, comme de paille menue.
(=Spiny Arcella, hemispherical, often deformed, spiny at the edge, with a shell composed of short rod-like fibers, like fine straw.)

(Translated from German into English, my comments in red:) This excellent form is on the whole rarer than the previous one (=Arcella vulgaris), but at times it is also common in Berlin. Since 1830, I have often found it again, especially numerous on June 26, 1834 and March 20, 1835, mostly at the same time as the others. Under every spine it seemed to me that there was a soft appendage (=pseudopod), as if it were under a half-cylinder, but I also see appendages like this, but only one at a time, on the opposite side. The spines are often only on one half of the edge of the disc, and the central shell opening is usually off the center. I saw 6 to 8 mostly curved spines, sometimes 3, 2 and 1. “Stomach cells” inside (=food vacuoles) were clearly recognizable, also filled with indigo, which is why the animal was counted among the safe “stomach animals” as early as 1830. During the charring, the form of the armor, formed from short, rigid fibers (=diatom frustules?) like chaff, was badly damaged. – Size up to the 1/18 Linie observed (without the spines) (=up to 111 µm, see my remarks on the Linie in Difflugia oblonga).

Explanation of the figures, Plate IX. Fig. VI.
3 individuals of different shapes are shown, all enlarged 300 times.
The sticks in Fig. A. are not swallowed naviculae but the fibers of the shell wall. The vesicles in between are “stomach cells” (=protoplasm). The large, bright, round spot is the lower shell opening.
Fig. B. has 3 spines, an opposite contractile process and middle “stomach cells”.
Fig. C. is an empty crooked bowl.
The latter 2 figures are not more homogeneous than the first in terms of the shell structure, but just not shown in the drawing. All have the spines on the left.