Opaline
The
opalines are a small group of peculiar
protists, found as endosymbionts in the gut of
frogs and toads. Each cell has two or more
nuclei, and is covered in short
flagella, arranged in rows. As such, they somewhat resemble
ciliates, but opalines have only one sort of nucleus, lack mouths, and divide longitudinally (between flagellar rows).
Opalina and
Protopalina are the best known genera.
Opalines were at one point considered possible ancestors of the ciliates, but the similarity between the two appears to be convergent evolution. They are now considered close relatives of the proteromonad flagellates, and the two
are included among the stramenopiles, though opalines may lack the tripartite hairs that are characteristic of that group.