Ultrastructural aspects of the ‘statocyst’ of Xenoturbella (Deuterostomia) cast doubt on its function as a georeceptor

The “statocyst” in the enigmatic worm Xenoturbella is a structure containing motile flagellated cells. It is situated inside the subepidermal membrane complex (between epidermis and muscular layers) in the anterior end of the body. The motile cells contain a lipophilic refractile body (“statolith”), and a series of vesicles from small dense core vesicles presumably formed from the refractile body to large vesicles with dense aggregates of filamentous tubules that become exocytized through secretion. It is unlikely that the statocyst is a georeceptor (true statocyst); maybe it has an endocrine function.