Ditrema longicollis (De Saedeleer, 1934)
Diagnosis: Shell composed of organic material, shaped like a half dome or an iglo; shell wall without any xenosomes, hyaline; young cells colorless, older cells pale yellow and later yellow brown; young shells are circular, older cells more irregular in shape. Shell with two opposite pseudostomes, each with a clear cylindrical neck, hence the name longicollis. Protoplast doesn€™t fill the shell fully, cell sometimes with epipodia connected to the shell. Periphery with up to twelve contractile vacuoles of different size. Granofilopodia present, usually three to five on each side. One pseudopodium runs straight out of each neck and branches into three to five granofilopodia, often at right angles. Filopodial granules clearly visible, but not evenly spaced; can differ strongly in density; spacing often at intervals between 5-12 µm. Cell and filopodia remain in contact with substrate or biofilm; immotile and solitary. Nucleus centrally, spherical with more or less irregular central nucleolus, sometimes surrounded by some small granules.
Dimensions: De Saedeleer: shell diameter without both pseudostomes 18.7-25.5 µm (n=23); neck 1.7-3.4 µm. My measurements: shell diameter without pseudostomes: 19.3-21.3 µm (n=6); neck 4.2-4.4 µm long; nucleus 4.9-6.8 µm, nucleolus 1.7-2.4 µm.
Ecology: feeds on small algae and probably bacteria. Fresh water. I found specimens in the biofilm on the surface of a fresh water pond. Belgium and the Netherlands.
Remarks: De Saedeleer considered this species to be a Diplophrys species and described it as Diplophrys longicollis (1934), but with that genus it has nothing in common, besides the two pseudostomes. The shell is half domed and seems to be double-layered.