So far I have only found Metacystis lagenula in the Simmelried. However, the species is very rare there. On average, I find a specimen about every 5 years. They are usually found between floating plant masses or detritus flakes.

In the literature, only the descriptions by Penard (1922) and Kahl (1935) seem to be available, whereby Kahl has adopted Penard’s drawing and description. I have not found any other descriptions of Metacystis lagenula.

Metacystis lagenula can be easily recognized by its Erlenmeyer flask-shaped lorica. The lorica is smooth and transparent. It is usually colorless or slightly brownish. At up to 71 µm, the loricae of my population were about 15 % larger than indicated by Penard (50–60 µm). The extended cells were also more than twice as long (up to 65 µm) as indicated by Penard (30 µm). Penard may have measured retracted specimens.

All specimens in my population contained symbiotic algae, which look similar to the Chlorella type with a cup-shaped chloroplast and have a diameter of 4.2–4.6 µm (s. fig. 3). I always found the macronucleus in the posterior third, with an attached lenticular micronucleus (s. fig. 6). There are 8–9 transverse rows of cilia on the body (s. fig. 4 b). According to Penard, there should be several caudal cilia at the posterior end. However, I could only recognize one at a time (s. fig. 5).

According to Penard’s description, Metacystis lagenula has 2 contractile vacuoles. The larger of the two is said to be located subterminally and the smaller in the anterior third near the oral funnel. However, I was only ever able to recognize the larger, subterminal contractile vacuole (s. fig. 4 a). Perhaps the anterior one was covered by the symbiotic algae or food vacuoles.