Development without Metamorphosis. The majority of the Nemertea, including the whole (?) of the Enopla, develop without a metamorphosis. The observations which have been made on this type are not very satisfactory, but appear to indicate that the formation of the hypoblast may take place either by invagination or by delamination.
Invaginate types have been observed by Barrois (No. [192]), Dieck (No. [196]) and Hubrecht.
Barrois’ fullest observations were made on Amphiporus lactifloreus (one of the Enopla), and those of Dieck on Cephalothrix galatheæ (one of the Anopla).
A regular segmentation is followed by a blastosphere stage with a small segmentation cavity. In Barrois’ type the inner ends of the cells of the blastosphere are stated to fuse into a kind of syncytium. A small invagination takes place, and the cells which take part in it separate from the epiblast, and then fuse with the syncytium within the blastosphere. Dieck finds that in Cephalothrix the invaginated mass simply vanishes.
Barrois’ statements about the fusion of the syncytium derived from the epiblast cells with the invaginated cells must be regarded as very doubtful. The formation of the germinal layers takes place, according to Barrois, by the separation of the internal mass of cells into mesoblast and hypoblast. The proboscis is formed, according to this author, from the mesoblastic tissues. Dieck, on the other hand, with greater probability, states that the proboscis is formed by an invagination. In Cephalothrix a further point deserves notice, in that the whole of the primitive epiblast becomes shed. In this fact there may perhaps be recognised the last trace of a metamorphosis like that in the type of Desor.
Delaminate types have been studied by Barrois (No. [192]) and Hoffman (No. [198]), both of whom give circumstantial accounts of their development.
Hoffman’s account is especially deserving of attention, since his observations were, to a great extent, made by means of artificial sections. The following account is taken from him. His observations were made on Tetrastemma varicolor, and Tetrastemma appears to be the genus in which this type of development has been most completely made out. After a regular segmentation the embryo forms a solid mass of cells, the outermost of which soon become distinguished as a separate epiblastic layer. At the same time the larva leaves the egg, and the epiblast cells become coated by an uniform covering of cilia. At the anterior extremity of the body is a bunch of long cilia; and at the hinder end two stiff bristles are formed, but soon disappear.
The internal mass of cells is still quite uniform, but as the larva grows in length the outermost of them arrange themselves as a columnar layer, constituting the mesoblast. Of the cells internal to the mesoblast the outer become columnar, and are converted into the walls of the alimentary tract, while the inner ones undergo fatty degeneration, and form a kind of food-yolk. In the later development the characters of the adult are gradually acquired without metamorphosis, and the larval skin passes directly into that of the adult. Both mouth and anus are formed nearly simultaneously by a rupture of the enteric wall from within. The nervous system arises as a thickening of the epiblast, which Hoffman states he has been able to see in sections. Hoffman also states that the epithelium of the proboscis is formed as a diverticulum of the alimentary tract, and that its sheath is formed by a special mesoblastic growth.
Barrois is less precise than Hoffman, from whom he differs in certain particulars. Hoffman’s statements about the proboscis are important if accurate, but require further confirmation.
Malacobdella. The early stages in development of the peculiar ectoparasitic Nemertine Malacobdella have been worked by Hoffman (No. [199]) by means of sections, and there appears to be a close agreement between the development of Malacobdella and that of Tetrastemma.