Many of the genera also have a gland situated in the gill cavity called a "gill gland", a secondary sex characteristic found in sexually mature male glandulocaudines that is apparently suited to release chemical signals. No genus contains species that have glands and other species without glands. This gill gland is derived from anterior gill filaments of the first gill arch. Gland size and degree of gill modification varies with species. Though the true function of the gill glands has yet to be determined, they are probably used to release chemical signals into the gill current.