The segmentation is uniform, and there is no trace of a segmentation cavity. On the third day after impregnation the outermost cells of the embryo become flattened and ciliated, and distinguished from the remaining spherical cells of the embryo as the epiblast. With the appearance of cilia a rotation of the embryo commences. On the fourth day the embryo becomes oval, and at one of the poles--the future anal pole--a separation takes place between the epiblast and the inner cells, giving rise to the body cavity. In it are a number of loose oval cells, which soon become stellate, and form a mesoblastic reticulum connecting the body wall and central cells of the embryo, which may now be spoken of as hypoblast. The body cavity increases in size, leaving at last the hypoblast and epiblast united only at one point--the oral pole--at which, on the fifth day, a crown of long cilia appears. The solid mass of hypoblast in the interior becomes differentiated into an outer layer of cells--the true glandular epithelium of the alimentary tract--and an inner core, the cells of which soon undergo fatty degeneration, and serve as food-yolk. The later stages of development, and the formation of the proboscis, etc., have not been worked out. General considerations. Of the types of larvæ hitherto found amongst the Nemertea, those with a metamorphosis, viz. the Pilidium type and that of Desor, are to be regarded as the primitive. But even in Pilidium there are evidences of a great abbreviation in development. Pilidium itself is probably a more or less modified ancestral form, while the peculiar development of the Nemertine within it is to be explained as a very much shortened record of a long series of changes by which the Pilidium became gradually converted into a Nemertine. The formation of the body wall of the Nemertine by four epiblastic invaginations is a remarkable embryological phenomenon, for which it is not easy to assign a satisfactory meaning; and it is probable that it is merely a secondary process of growth similar to the formation of imaginal discs in the larvæ of Diptera (vide Chapter on Tracheata), which has had its origin in the abbreviation of the development just alluded to. The development of the type of Desor is clearly a simplification of the Pilidium type, and its peculiarities are to be explained by the fact that the first larval form has no free existence. The types without metamorphosis have no doubt a development of a still more simplified character; they are remarkable however in presenting us, if the existing descriptions are to be trusted, with examples of delamination and invagination coexisting in closely allied forms. TREMATODA.