Fig. 225. Two chætopod larvæ. (From Gegenbaur.)
o. mouth; i. intestine; a. anus; v. præoral ciliated band; w. perianal ciliated band.
In the first place, this ancestral form, of which [fig. 231] A is an ideal representation, would appear to have had a dome-shaped body, with a flattened oral surface and a rounded aboral surface. Its symmetry was radial, and in the centre of the flattened oral surface was placed the mouth, and round its edge was a ring of cilia. The passage of a Pilidium-like larva into the vermiform bilateral Platyelminth form, and therefore it may be presumed of the ancestral form which this larva repeats, is effected by the larva becoming more elongated, and by the region between the mouth and one end of the body becoming the præoral region, and by an outgrowth between the mouth and the opposite end developing into the trunk, an anus becoming placed at its extremity in the higher forms.
Fig. 226. Polygordius larva.(After Hatschek.)
m. mouth; sg. supraœsophageal ganglion; nph. nephridion; me.p. mesoblastic band; an. anus; ol. stomach.
If what has been so far postulated is correct, it is clear that this primitive larval form bears a very close resemblance to a simplified free-swimming Cœlenterate (Medusa), and that the conversion of such a radiate form into the bilateral took place, not by the elongation of the aboral surface, and the formation of an anus there, but by the unequal elongation of the oral face, an anterior part, together with the dome above it, forming a præoral lobe, and a posterior outgrowth the trunk ([figs. 226] and [233]); while the aboral surface became the dorsal surface.
This view fits in very well with the anatomical resemblances between the Cœlenterata and the Turbellaria[141], and shews, if true, that the ventral and median position of the mouth in many Turbellaria is the primitive one.
Fig. 227. Larva of Echiurus. (After Salensky.)
m. mouth; an. anus; sg. supraœsophageal ganglion (?).