1GG 



NEMERTEA. 



terioriy. Round the edge of the ventral surface a ciliated band 

 makes its appearance. 



Two pairs of invaginations 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 iuvagination 

 forms a sack, the outer 

 wall of which becomes 

 very thin and the in- 

 ner wall (corresponding 

 with the whole invagi- 

 nation of the type of 

 Desor) very thick. The 

 inner walls of the four 

 thickenings, which I 

 may speak of as discs, 

 now fuse together, each 

 disc first uniting with 

 its fellow, and finally 

 the two pairs uniting. 



A ventral germinal 

 pi ate is thus estab- 

 lished, which gradually 

 grows round the intes- 

 tine of the Pilidium to 

 form the skin of the 

 future Nemertine. The 

 outer thin layer of each 

 of the discs grows pari 

 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 by a ciliated canal 1 . The proboscis arises as an 



FIG 94. 



A. PILIDIUM WITH AN ADVANCED NEMERTINE WOKM. 



B. ElPE EMBKYO OF NEMERTES IN THE POSITION IT 



OCCUPIES IN PILIDIUM. (Both after Biitschli.) 



a', cesophagus ; st . stomach ; /. intestine ; pr. pro- 

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

 svsteni. 



1 This is the view of both Metschuikoff (No. 202) and Leuckart and Pagenstecher 

 (No. 201), and is further confirmed by Barrois, but Biitschli (No. 193), though he has 

 not observed the earliest stages of their outgrowtli, believes them to be invaginations 

 of the Nemertine skin. 



