The cells originally split off from the discs undergo considerable changes, some of them arrange themselves around the proboscis as a definite membrane, which becomes the proboscidean sheath, some also form a true splanchnic layer of mesoblast, and the remainder, which are especially concentrated during early embryonic life in the anterior parts of the body, form the general interstitial connective tissue. The cephalic ganglia are 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 larvæ. 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.).
Fig. 93. Two stages in the development of Pilidium.
(After Metschnikoff.)
ae. archenteron; œ. œsophagus; st. stomach; am. amnion; pr.d. prostomial disc; po.d. metastomial disc; c.s. cephalic sack.
The primitive alimentary tract so formed becomes divided into œsophageal and gastric regions ([fig. 93] B, œ. 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 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 invagination 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.
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 invagination forms a sack, the outer wall of which becomes very thin and the inner wall (corresponding 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 together, each disc first uniting with its fellow, and finally the two pairs uniting.