Microthorax viridis is one of the most common ciliates in the mud from Simmelried. The species is present in almost every sample. Despite its small dimensions of about 40 µm, Microthorax viridis can be recognized by its green color and characteristic shape even at low magnifications. Kahl gives only a very short description of Microthorax viridis and only a rough drawing of the left side (s. drawing above). On the right side (fg. 1 a-b and 2 a-b) 3 rows of cilia are visible, which are all interrupted in the middle of the body (s. fig. 1a). To my knowledge the left side has not been examined and described yet. There I could observe and document 5 pores with one cilium each (s. fig. 3 a-b). As a consequence two pores are in the anterior third and three pores in the posterior half. The number of symbiotic algae I could determine to be 40–60. Mostly there were about 50 symbiontic algae. The symbiotic algae have a diameter of 5–7 µm and have their own nucleus (s. fig. 4). They seem to be of Chlorella type. I could detect some spindle-shaped extrusomes with a length of 6 µm below the pellicle (s. fig. 5 a-b). However, the number of extrusomes per cell was < 10.