Brachonella cydonia first described as Metopus cydonia by Kahl in 1927. Later Jankowski (1964) transferred the species to the genus Brachonella. For the genus Brachonella, Jankowski defines the following characteristics:

– anterior end (dome) larger than the posterior end.
– the adoral zone describes a helical line when it runs to the mouth opening

I find Brachonella cydonia rarely but regularly. So far all findings are from the Simmelried. The species is difficult to photograph, because of the high coverslip sensitivity. Already Kahl noted: “…kollabiert sofort beim Auflegen des Deckglases… (collapses immediately when the coverslip is put on)”. I can confirm this behavior, which is why I have not been able to examine any squashed specimen in detail. All specimens shown below were photographed at high layer thickness.

As the only deviation from Kahl’s description, I found the macronucleus to be often spherical, although Kahl himself was not sure if the elongated macronucleus is a constant feature. Kahl describes that the tuft of long caudal cilia can be actively spread. I have not been able to observe this with my specimens photographed swimming. Possibly this process can be seen only in resting specimens.