168 NEMERTEA. 



tions 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 bi-istles 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 diverticulurn 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 ecto- 

 parasitic 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. 



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 remain- 

 ing 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 alimentaiy 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 larvre hitherto found 

 amongst the Nemertea, those with a metamorphosis, viz. the Pilidium 



