Difflugia curvicaulis Penard, 1899
Diagnosis: Test tubular, narrowing from the fundus to the aperture, usually transparent, colorless or brown, with a distinct aboral horn, often curved and sometimes perforated at the apex; test usually with smooth outline, composed of small and medium sized pieces of flat quartz and fragments of diatoms, embedded in an organic cement which forms a regular network; irregular thicker pieces of quartz as common in most other Difflugia species, are rare. Aperture circular, surrounded by regular arrangement of small particles. Nucleus spherical, with some irregularly shaped nucleoli laying close to the nuclear membrane.
Dimensions: Penard 170-200 µm (without horn); Length 146-232 µm; my measurements: 197-240 µm (including horn); nucleus 25-27 µm, nucleoli c. 6-8 µm.
Ecology: freshwater; sediments of ditches and lakes.
Remarks: According to Penard (1899) D. curvicaulis differs from D. acuminata and D. elegans in the smoothness of the shell and the curved aboral horn. This species was considered by Cash (1909) to differ from D. acuminata only in the shape of the terminal horn. Mazei and Warren (2012) consider this form as a junior synonym of D. acuminata. The main distinguishing character however is the structure of the nucleus. Penards description is quite clear, though his drawing is confusing. This drawing shows the surface of the nucleus, giving the impression that the nucleus is filled with nucleoli.
I have found shells of this species between typical D. acuminata shells. They could be distinguished from the latter by their transparancy, caused by transparent and flat xenosomes and the absence of thicker sand graines, and the structure of the nucleus. A large part of these flat xenosomes were fragments of diatoms.