200 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 Pilidium 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 st.}. 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 



FlG. 93. TWO STAGES IN THE DEVELOPMENT OF PlLIDIUM. (After Metschnikoff.) 



ae. archenteron ; <r. oesophagus ; st. stomach ; a in. 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. 



