A: Morphological phylogeny.

B: Molecular phylogeny.
-In the traditional phylogeny (A), the phylum Platyhelminthes is depicted as a sister taxon to the rest of the protostome phyla, and as having diverged earlier from the lineage that led to the rest of the protostomes. In the molecular phylogeny (B), Platyhelminthes is depicted as a lophotrochozoan phylum. What probably led to this change?
A) Platyhelminthes ceased to be recognized as true protostomes.
B) The removal of the acoel flatworms (Acoela) from the Platyhelminthes allowed the remaining flatworms to be clearly tied to the Lophotrochozoa.
C) All Platyhelminthes must have a well-developed lophophore as their feeding apparatus.
D) Platyhelminthes' close genetic ties to the arthropods became clear as their Hox gene sequences were studied.

Question

A: Morphological phylogeny.
  
B: Molecular phylogeny.
-In the traditional phylogeny (A), the phylum Platyhelminthes is depicted as a sister taxon to the rest of the protostome phyla, and as having diverged earlier from the lineage that led to the rest of the protostomes. In the molecular phylogeny (B), Platyhelminthes is depicted as a lophotrochozoan phylum. What probably led to this change?
A) Platyhelminthes ceased to be recognized as true protostomes. 
B) The removal of the acoel flatworms (Acoela) from the Platyhelminthes allowed the remaining flatworms to be clearly tied to the Lophotrochozoa. 
C) All Platyhelminthes must have a well-developed lophophore as their feeding apparatus. 
D) Platyhelminthes' close genetic ties to the arthropods became clear as their Hox gene sequences were studied.

Quizplus · Accepted Answer

The molecular phylogeny shows Platyhelminthes as a lophotrochozoan phylum, which suggests that they are closely related to other protostomes like the mollusks, annelids, and brachiopods. This change was likely due to the removal of the acoel flatworms from the Platyhelminthes, which allowed the remaining flatworms to be more clearly associated with the Lophotrochozoa based on molecular data.

A: Morphological Phylogeny. B: Molecular Phylogeny