In two of these families, which were more especially examined by Muller, and which are nearly related to each other, the species agree most closely in all important characters: namely in their sense organs, circulating systems, in the position of the tufts of hair within their complex stomachs, and lastly in the whole structure of the water-breathing branchiae, even to the microscopical hooks by which they are cleansed.
Familia horietarik bitan, Müllerrek berezikiago aztertuak eta elkarren oso antzekoak berauek, espezieak elkarren antz-antzekoak dira ezaugarri inportante guztietan, zehazki, zentzumen-organoetan, zirkulazio-aparatuan, beren urdail konplexuaren barruan ile multzoak duten kokagunean, eta azkenik ur-arnasketarako dituzten brankien egitura osoan, are garbitzeko erabiltzen dituzten gako mikroskopikoetan beretan ere.