The nephridia of the adult Asymmetron are described for the first time. They are of the protonephridial type, and resemble those of Branchiostoma, though smaller and simpler in structure. The canal of the paired nephridia is in the shape of a triangular flattened sac into which open numerous solenocytes. The sac and the solenocytes extend over the inner wall of the suprapharyngeal coelom, and the external pore opens into the atrium near the top of the secondary bar. The first pair of nephridia, however, is situated in the epipterygial coelom. The paired nephridia are ‘retroperitoneal’ ; but the coelomic epithelium is interrupted, and does not cover the solenocyte field, so that coelomic fluid bathes the solenocyte tubes.
The nephridium of Hatschek of Asymmetron resembles that of Branchiostoma, but the diverticula are shorter.
The nephridia of the two genera are compared, and certain details in the structure of those of Branchiostoma are discussed.