The histology of the seminal vesicles of Sagitta serratodentata, S. pseudoserratodentata, S. pacifica, S. tasmanica, and S. bierii is described. To conclude from the structure of these vesicles S. serratodentata, S. pseudoserratodentata, and S. pacifica are closely related. For both S. tasmanica and S. bierii no relations with other species could be traced. S. bierii is considered the most primitive species of the group.