LARVAL FORMS : THEIR NATURE, ORIGIN AND AFFINITIES. 395 
end of the body becoming the prseoral region, and that 
between the mouth and the opposite end developing into 
Fig. 13. — Polygordius Larva. (After Hatschek.) mouth; 5^. supra- 
oesophageal ganglion ; nph. nephridion ; me.p. mesoblastic band ; 
an. anus ; ol. stomach. 
the trunk, an anus becoming placed at the extremity of 
the trunk in the higher forms. 
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 elonga- 
tion of the oral face, an anterior part forming a preeoral 
lobe, and a posterior part the trunk ; while the aboral 
surface became the dorsal surface. 
This view fits in very well with the anatomical resem- 
liG. 14. Larva of Echiurus. (After Salcnsky.) mouth ; anus 
sfj. supra-oesophageal ganglion (?). 
blanccs between the Coelenterata and the Turbellaria,^ and 
* Vide ‘ A Treatise on Comparative Embryology,’ vol. i, p. 148 and 158. 
In this connection attention may be called to Coleoplana Metschnikowei, 
