THE NEMERTINI 



167 



Fid. VI. 



Three figures to illustrate the formation of the young Nemertine within the Pilidium (from 

 Burger). They are diagrammatic sections (or projections) across the larva, just above the origin 

 of the lappets. 1 shows the origin of the two pairs of imaginal discs, the pair of excretory pits 

 (from the stornodaeum), and the median proboscis pit. In each of the imaginal discs the bottom 

 becomes thickened and the sides thin. In 2 the imaginal discs are closed sacs (amniotic sacs) ; 

 each is flattening and extending round the enteron of the larva, the cerebral organ is making its 

 appearance. 3, the imaginal discs have reached their fullest development ; the new mesoblast 

 has been given off, and the body wall of the Nemertine established. The dorsal and ventral 

 ganglia are denned (the cephalic "plates" are left unshaded to show these), and the excretory 

 organ is branched. Am, amnion ; Amh, amniotic cavity ; C, cerebral organ ; D, enteron ; Dg, 

 dorsal ganglion ; Ep, definitive epidermis of young Nemertine ; Kp, cephalic plate, the thickened 

 floor of the cephalic pit ; Ks, cephalic pit, or imaginal disc ; N, nephridium, or its foundation ; 

 Oe, larval stomodaeum ; R, proboscis ; Ra, invagination to form proboscis ; Re, the foundation of 

 the rhynchocoel, its muscular wall, and the muscular coat of the proboscis ; Rp, somatic plate, 

 the thickened floor of the somatic pit ; Rs, somatic pit or imaginal disc ; S, lateral nerve stem 

 of Nemertine ; Vg, ventral ganglion. 



