2OO NEMERTEA. 



the walls of the body, and are only prevented from extending forwards by 

 a membrane of connective tissue. This mass gradually forms itself into a 

 distinct canal, lined by columnar cells. 



Pilidium. In the case of the true Piltdium type, the larva is 

 hatched very early and leads the usual existence of surface 

 larvae. A regular segmentation is followed by an invagination 

 which does not however cause the complete obliteration of the 

 segmentation cavity (fig. 93 A, a.e.). 



The primitive alimentary tract so formed becomes divided 

 into cesophageal and gastric regions (fig. 93 B, oe. and .$/.). Even 

 while the invagination of the archenteron is proceeding, the 

 larva becomes ciliated throughout, and assumes a somewhat 

 conical form, the apex of the cone being opposite the flat ventral 

 surface on which the mouth is situated (fig. 93, A and B). From 



FIG. 93. Two STAGES IN THE DEVELOPMENT OF PILIDIUM. (After Metschnikoff.) 

 of. archenteron ; <r. oesophagus ; st. stomach ; am. amnion ; pr.d. prostomial disc ; 

 PO. d. metastomial disc; c.s. cephalic sack. 



the apex a flagellum projects in many forms, giving the larva a 

 helmet-like appearance. In other forms a bunch of long cilia 

 takes the place of the flagellum (fig. 94), and in others again the 

 flagellum is not represented. After the completion of the inva- 

 gination a lobe grows out on each side of the mouth, and less 

 well developed lobes may appear anteriorly and posteriorly. 

 Round the edge of the ventral surface a ciliated band makes its 

 appearance. 



