Stylophoran echinoderms are known for displaying a variety of lamellar structures formed by skeletal elements. Most of them have been interpreted as respiratory structures, the lamellar shape allowing a more efficient transfer of ambient oxygen into the animal’s tissues. While most of these structures are external (e.g. lamellipores of scotiaecystid cornutes), the "ctenoid organ" of the mitrate Lagynocystis is unique among stylophorans in that it is internal (intrathecal). The “ctenoid organ” extends from the inner roof of the theca as several slightly curved slats, arranged in three bundles: a central one, and two lateral ones. During the ontogenetic development of the organism, new slats grow in between those already existing. The identification of exceptionally preserved soft tissues in stylophorans indicates that the “ctenoid organ” would have separated the oesophagus, situated in the proximal rings of the aulacophore, from the rest of the digestive tract, located in the theca. This particularity makes the hypothesis of a respiratory function unlikely, because the “ctenoid organ” was not in direct contact with the external seawater. Other unusual features of Lagynocystis are its frequent association with blind trilobites, and its surprisingly long stratigraphic range, from at least the late Tremadocian (possibly the Furongian) to the late Sandbian. These characteristics link Lagynocystis to an animal living in deep seawater, which could also explain its peculiar anatomy. Hence, in this study, we test the novel hypothesis that the “ctenoid organ” of Lagynocystis could serve as a hosting structure for chemosynthetic bacteria colonies. Such structures are known as a convergent trait among several extant taxa such as bivalve molluscs, decapod arthropods, and polychaete worms, living at various depths of marine environments, although they are more often associated with deep-sea hydrothermal vent systems. This kind of symbiosis has been suggested once in the ancient fossil record, for olenid trilobites that lived in putatively low-oxygen, high-sulfur waters. Lagynocystis may be another ancient example of such adaptation to deep-sea environments, as well as the first occurrence of a chemoautotrophic symbiosis among echinoderms.