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THE BILATERIANS ARE split into two clades, the protostomes and the deuterostomes. These are differentiated based upon their embryonic development. While there are several differences (and variation within each clade!), the key difference is the fate of the blastopore. I was once presented with the question, “In protostomes the mouth develops from the blastopore, but in deuterostomes the anus develops from the blastopore, so how can you evolve a deuterostome from a protostome?” The protostomes include the insects and annelid worms, how can you reverse the development of those to get to deuterostomes like frogs and starfish?

The short answer is, you don’t! Let me count the ways:

  1. You might not have to in the first place–deuterostomes and protostomes might have independent origins, or deuterostomy might be the ancestral condition.
  2. The ancestral bilaterian would not have been as complicated as extant organisms, so messing around with its development would have been easier.
  3. This depiction of gastrulation is an oversimplification. Some protostomes produce both the mouth and the anus from the blastopore. If this is the primitive condition, deuterostomes just have to make a second mouth and close the first.

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TODAY I WAS reading about protostomes and deuterostomes, and in the process stumbled across a mention of chaetognaths as deuterostome-like protostomes. Since I had no clue what a chaetognath is, I looked it up and found Wikipedia’s entry, which describes them as “predatory marine worms”. This prompted the question: What is a worm?

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