PLATYELMINTHES. 



201 



Two pairs of imaginations of the skin, just as in the type of 

 Desor, now make their appearance, one pair in front of and the 

 other behind the mouth (fig. 93 B, pr.d. and po.d.}, and each of 

 them by the closure 

 of the opening of 

 invagination forms 

 a sack, the outer 

 wall of which be- 

 comes very thin 

 and the inner 

 wall (correspond- 

 ing with the whole 

 invagination of the 

 type of Desor) very 

 thick. The inner 

 walls of the four 

 thickenings, which 

 I may speak of as 

 discs, now fuse to- 

 gether, each disc 

 first uniting with 

 its fellow, and 

 finally the two 

 pairs uniting. 



A ventral ger- 

 minal plate is thus 

 established, which 

 gradually grows 

 round the intestine 

 of the Pilidium to 

 form the skin of 

 the future Nemer- 

 tine. The outer 

 thin layer of each of the discs grows part passu with the inner 

 layer, and furnishes an amnion-like covering for the embryo 

 which is forming within the Pilidium (fig. 94, an}. 



In connection with the young vermiform Nemertine there is 

 formed on each side an outgrowth from the oesophagus (fig. 94) 

 which is eventually placed in communication with the exterior 



FIG. 94. 



A. PILIDIUM WITH AN ADVANCED NEMERTINE 

 WORM. 



B. RIPE EMBRYO OF THE NfiMERTEA IN THE 

 POSITION IT OCCUPIES IN PlLIDIUM. (Both after 



Butschli.) 



&. oesophagus ; st. stomach ; i. intestine ; pr. pro- 

 boscis ; Ip. lateral pit ; an. amnion ; n. nervous system. 



