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As I unerstand it, at some point an embryo develops a hole; this hole continues to punch through, and eventually makes an opening at the other side of the embryo. If the original hole becomes the mouth, then the organism is called a protostome. If the second opening becomes a mouth, then the animal is called a deuterotome.

My first question is, is this distinction made only for organisms who have two openings? In other words, would an organism with only one orifice be called a protostome, or would it be neither?

Why is the protostome/deuterostome distinction important? Why does it matter whether embryo develops a mouth from the first or second orifice?

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The terminology is really just describing the process in embryonic development that you describe: deuterostome means "second mouth"; protostome means "first mouth."

The significance of these groups is in evolutionary history and cladistics. All of the deuterostomes have a shared common ancestor that is not shared with any protostomes, and vice versa. Regardless of developmental pattern or number of openings during development, no other organism should be given these names (and if it is, and the correct ancestry is determined later, they would be moved out of those categories accordingly).

This is actually really important in biological classification of organisms, and before modern genetic techniques these types of shared characteristics were the best way to determine shared ancestry. Think a bit about what it means for an organism with two openings to differentiate one as mouth and the other as anus: once they are strongly differentiated, there is a huge barrier towards going "backward" and "switching" openings because mouth and anus have very different functions. However, if you go from having one or no openings to having two, before there is specialization, the distinction of which one develops first isn't likely to be that advantageous one way or the other.

Nephrozoa contains the deuterostomes and protostomes, consisting of almost all extant animals. However there are also other animals that are neither. For example, Xenacoelomorpha might be the closest thing to a bilateral animal with only one orifice, though they don't really have a true gut at all.

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  • $\begingroup$ Thanks for the answer; if I could as a follow up question: In my class we are just focusing on 6 phyla of animals: Porifera, Chordata, Mollusca, Cnidaria, Arthropoda, Echinodermata. Is it possible to say that some of these phyla are completely composed entirely of protostomes, and some of deuterostomes? $\endgroup$ – Ovi Oct 2 '18 at 23:11
  • $\begingroup$ @Ovi I can't really help you with your homework more than I already have, but if you see the Wikipedia pages for deuterostomes and protostomes (and most likely your textbook as well...) the answer to your comment question should be quite apparent. $\endgroup$ – Bryan Krause Oct 2 '18 at 23:28
  • $\begingroup$ Thanks for all your help; I will check the wikipedia article. Anyways it's not a homework question, I am studying for a test. My book is very short (written by professor specifically for this class) and doesn't seem to answer this question. $\endgroup$ – Ovi Oct 2 '18 at 23:32

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