Simple SummaryRare organisms represent a challenge for researchers in all fields of study in biology. In the realm of ciliatology, the generaDactylochlamysandLegendreaare considered to be such cases. Very little information has accrued in over a century since their first descriptions; only a few published reports, online images or videos of rarely encountered individual specimens exist.DactylochlamysandLegendreaare also morphologically remarkable for their likely independently evolved tentacle-like structures. Recently, the first molecular data were published for the speciesL. loyezae. In our study, we present more robust phylogenetic analysis based on three molecular markers ofDactylochlamys pisciformisand all of the three knownLegendreaspecies, showing that they likely represent a new anaerobic lineage of ciliates. We first provide a detailed morphological characterization of both genera using modern microscopy and staining methods. We identify and discuss the bacterial (Syntrophaceae) and archaeal endosymbionts harbored by both genera based on 16S rRNA gene sequences. We also discovered thatLegendreapreys on gastrotrichs, which is supported by molecular data and a unique video material documenting the feeding behavior of oneLegendreaspecies. This study brings essential information needed to better understand the phylogeny, life strategies, and rarity of these organisms and emphasizes the importance of citizen science.AbstractFree-living anaerobic ciliates are of considerable interest from an ecological and an evolutionary standpoint. Extraordinary tentacle-bearing predatory lineages have evolved independently several times within the phylum Ciliophora, including two rarely encountered anaerobic litostomatean genera,LegendreaandDactylochlamys. In this study, we significantly extend the morphological and phylogenetic characterization of these two poorly known groups of predatory ciliates. We provide the first phylogenetic analysis of the monotypic genusDactylochlamysand the three valid species ofLegendreabased on the 18S rRNA gene and ITS-28S rRNA gene sequences. Prior to this study, neither group had been studied using silver impregnation methods. We provide the first protargol-stained material and also a unique video material including documentation, for the first time, of the hunting and feeding behavior of aLegendreaspecies. We briefly discuss the identity of methanogenic archaeal and bacterial endosymbionts of both genera based on 16S rRNA gene sequences, and the importance of citizen science for ciliatology from a historical and contemporary perspective.