According to Heymons, the yolk-cells must be Metameric Segmentation.
It is important to notice that the metameric plan of growth of Chaetopods is still preserved.
In one family (Ligulidae) the segmentation is only expressed in the metameric distribution of the generative organs and the worm is externally unisegmental.
In the latter respect, and in the fact that they frequently develop by a metamorphosis, they approach the Mollusca, but they differ from that group notably in the occurrence of metameric segmentation affecting many of the systems of organs.
Palaeontology therefore throws no light on the question whether the metameric or the unsegmented Mollusca were the earlier.