Mesodinium pulex is a common ciliate that can be found especially in old samples with decomposing plant material. I was able to detect it in almost all of sampling sites.

Mesodinium pulex has a characteristic swimming style. Between very fast jumps, the ciliate rests vertically and starts to spin. This is followed by further jumps at a very high swimming speed.

Mesodinium pulex is somewhat similar in shape and size to Urotricha agilis, but the ciliation of Mesodinium pulex is restricted to the equatorial zone, where there is a pre-equatorial wreath of cirri and an equatorial wreath of cilia (s. fig. 1 a). In addition, Mesodinium pulex has no caudal cilia and there are often retractable tentacles around the pharynx. The distal ends of the tentacles were interpreted by Kahl as funnels (s. drawing 4 , above). However, I could clearly see that the ends are finely split (s. fig. 1 a). I have found specimens with outstretched and retracted tentacles (s. figs. 1 a-d and 3 a-c). The function of the tentacles is unknown.

Penard gave the number of macronuclei as two in 1922. However, this observation was never confirmed. I was also unable to determine the number myself. If Mesodinium pulex is squashed, the cell bursts and disintegrates very quickly. There is only one contractile vacuole, which is terminal (s. fig. 2 b).