The immediate allies of the Nemerteans are evidently the Planarians, and Ehrenberg's class Turbellaria, as now amended, is therefore a very natural one. There is, however, a considerable divergence between the two groups; and since it is only through the Planarians that the Nemerteans can in any way be linked on to the Trematoda, it is found that there is little structural resemblance between the two last-mentioned types. The various parts of the organism in the Trematodes and Planarians (through which the connection of the Nemerteans with the former may be supposed to occur) were contrasted, and their homologies explained. The structural relations of the Planarians and Nemerteans were then considered; and, thirdly, the connections of the latter with the higher Annelids were alluded to.