1 66 



THE NEMERTINI 



bottom is formed of columnar cells, and constitutes the "germ 

 disc " or imaginal disc (cf . those of some insects). The outer walls 

 consist of a flat epithelium. The aperture of invagination closes, 

 and the pit becomes a sac, the thin wall of which is the " amnion " 

 (cf. vertebrates and insects). These four flattened sacs sink in- 

 wards and extend laterally in all directions till they finally meet, 



Fro. V. 



The fully developed Pilidium, bearing the young Neinertine within its amniotic sac (after 

 Korschelt and Heider). Am, amnion ; D, enteron of Pilidium and Nemertine ; EC, ectoderm 

 of Neinertine ; M, mouth of Pilidium ; JV, nervous system of Nemertine ; R, proboscis ; So, 

 excretory organ. 



thus embracing the larval enteron and a certain amount of meso- 

 blast, and giving rise to the body wall of the young Nemertine 

 (Figs. V., VI.). 



From the inner surface of the imaginal discs new mesoblast arises, 



thus a new body wall is formed within the larval skin. A fifth pit has 



meanwhile made its appearance at the anterior end of the larva, above 



the ciliated band ; instead of flattening out as the others do, it elongates 



ackwards to form a tube ; it is the forecast of the proboscis (Ra). The 



