The ascidiozooids in all but the youngest colonies lack any definite arrangement and are densely packed (Detail of an ascidiozooid, Pyrosoma aherniosum 1, Pyrosoma aherniosum 2). Both protogynous and protandrous conditions may occur. Colonial wall provided with longer or shorter, conical or truncate processes (Pyrosoma ovatum 1), never entirely smooth (sketch of a colony, Pyrosoma ovatum, Pyrosoma aherniosum).
Van Soest (1981) recognized four species. Three of them were recorded from the South Atlantic Ocean: Pyrosoma atlanticum, Pyrosoma aherniosum, and Pyrosoma ovatum. The fourth species, Pyrosoma godeauxi Van Soest, 1981, is known only from its original citation from the Indian Ocean.