PLATYELMINTHES. 165 



stated to become gradually differentiated in the prostomial mesoblast, and 

 the two cords connected with them in the metastomial mesoblast. 



At the time when the larval skin is cast off the original mouth becomes 

 closed, and it is not till some time afterwards that a permanent mouth is 

 formed in the same situation. During the early part of embryonic life 

 the intestine is lined with columnar cells, but, before the loss of the larval 

 skin, the walls of the intestine undergo a peculiar metamorphosis. Their 

 cells either fuse or become indistinguishable, and their protoplasm appears to 

 become converted into yolk-spherules, which fill up the whole space within 

 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 the apex a flagellum pro- 

 jects 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 invagination a lobe grows out on each side of the 

 mouth, and less well developed lobes may appear anteriorly and pus- 



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

 ae. archenterou ; oe. oesophagus; st. stomach; am. amnion ; pr.d. prostomial disc ; 

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



