I find Phalansterium digitatum frequently and regularly between floating aquatic plants or on the top layer of mud. In old samples, the species sometimes reproduces strongly and also likes to settle on the floating coverslip.

I usually find clearly branched colonies (s. fig. 1). Only rarely have I also found free-floating, spherical colonies (s. fig. 2). The finger-shaped gelatinous tubes are interspersed with approx. 2 µm large grains, which are often brownish in color. They are excreted by the flagellates themselves. They are not detritus.

Some characteristics of Phalansterium digitatum appear to be variable. Various authors have observed either one or sometimes two contractile vacuoles. In my population there were usually two (s. fig. 4 a-b). The number of flagella also seems to be variable. Cienkowsky (1870) and Skuja (1964) have already noted that the cells in some colonies have only one flagellum and in other colonies two. I can confirm this observation, as I have also observed cells with one flagellum (s. fig. 2) but also with two flagella (s. fig. 4 a-b).

This variability of characteristics makes it difficult to distinguish it from the similar species Spongomonas uvella. This species also forms branched colonies whose tubes are interspersed with brownish granules. However, the tubes are said to be shorter and not so strongly branched. According to Jeuck & Arndt (2014), the absence of a collar at the base of the flagella, which only Phalansterium digitatum possesses, remains a reliable distinguishing feature.