Trithigmostoma cucullulus is a very common ciliate that I find in almost all my localities. It is easy to observe as it likes to settle on the floating coverslip.

Trithigmostoma cucullulus has the typical structure of a cyrtophorid ciliate. The body is dorso-ventrally flattened. The dorsal side is weakly convex and naked with the exception of an anterior dorsal brush (s. figs. 1 b and 3). In contrast, the ventral side is flat and ciliated. In the anterior third the mouth opening with an oral basked is located. A pre-oral kinety extends from the mouth opening to the beak-shaped bulge of the body on the left side at the anterior end. The shape of this pre-oral kinety is an important identifying feature of cyrtophorid ciliates. In the genus Trithigmostoma it is continuous, without interruption. In addition, there is no cilium-free field in the middle of the ventral side, as is the case in the genus Chilodonella, for example. In Trithigmostoma cucullulus, the ventral somatic kinetes run approximately parallel and all end below or above the pre-oral kinety. This results in the typical cilia pattern of cyrtophorid ciliates (s. fig. 4).

Trithigmostoma cucullulus feeds mainly on diatoms, cyanobacteria and small algae, which are often gutted en masse. The cytoplasm contains 2–11 contractile vacuoles. In my population there were usually 5–6 (s. fig. 1 a). The macronucleus is ellipsoid with a central nucleolus. The micronucleus is close to the macronucleus (s. fig. 5).

The body of Trithigmostoma cucullulus is covered by an alveolar layer about 1 µm thick. This has a mucus-like character and has a very conspicuous, reticulate pattern (s. figs. 2 b and 6). This can be seen particularly well on the dorsal side. The alveolar layer can be shed and then often shows gaps.

More images and information on Trithigmostoma cucullulus: Michael Plewka-Freshwater life-Trithigmostoma  culucullulus