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 Coelenterate (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. [FIG. 226. POLYGORDIUS LARVA. (After Hatschek.) _m._ mouth; _sg._ supraoesophageal ganglion; _nph._ nephridion; _me.p._ mesoblastic band; _an._ anus; _ol._ stomach.] This view fits in very well with the anatomical resemblances between the Coelenterata and the Turbellaria[141], and shews, if true, that the ventral and median position of the mouth in many Turbellaria is the primitive one. [141] _Vide_ Vol. II. pp. 179 and 191. In this connection attention may be called to _Coeloplana Metschnikowii_, a form described by Kowalevsky, _Zoologischer Anzeiger_, No. 52, p. 140, as being intermediate between the Ctenophora and the Turbellaria. As already mentioned, there does not appear to me to be sufficient evidence to prove that this form is not merely a creeping Ctenophor. [FIG. 227. LARVA OF ECHIURUS. (After Salensky.) _m._ mouth; _an._ anus; _sg._ supraoesophageal ganglion (?).] [FIG. 228. DIAGRAM OF A LARVA OF THE POLYZOA. _m._ mouth; _an._ anus; _st._ stomach; _s._ ciliated disc.] The above suggestion as to the mode of passage from the radial into the bilateral form differs largely from that usually held. Lankester[142], for instance, gives the following account of this passage: [142] _Quart. Journ. of Micr. Science_, Vol. XVII. pp. 422-3.