Recently, there was found in Spain a shelled Pre-Cambrian Critter which showed distinct evidence of a gut.  This got me thinking about the distinction between the deuterostomes and protostomes, interest in which was sparked again by the even more recent article from National Geographic:

Giant Sea Cucumber Eats With Its Anus

And here it is, the giant California Sea Cucumber Parastichopus californicus:

Now the Sea Cucumber is one group of the Echinoderms, a phylum which along with the Chordates (of which humans make up one genus) and the Hemichordates constitute the Deuterostomes, a select group of creatures in whose embryonic development the first opening to appear, called the blastopore, develop into the anus which therefore appears before the mouth.  As Nicholas Horton so elegantly phrased it: It’s True, We’re all Butt Heads.

This makes me wonder, what exactly were the feeding habits of the ancestral Deuterostome?  Was it an argument over table manners in the late Ediacaran which propelled the differentiation of the Deuterostomes from all the rest, the Protostomes?

Invertebrate biology text books tell us little about this.   Although the 1987 Animals Without Backbones: An Introduction to the Invertebrates (Third Edition) by Buchsbaum×2 and Pearse×2 gives a very good introduction to the vastly different embryology of the two groups, the word “deuterostome” hardly gets a look-in, and not much more in Living Invertebrates by the same four authors (same year).

Was it simply that, while the morphology of the embryos was well understood back in the 60s or before, the actual sequence of their development was only worked out later?  Hardly — a dictionary search shows the word “deuterostome” has appeared in the 50s, and the complexities of development are already apparent in Dynamics in Metazoan Evolution: the Origin of the Coelom and Segments by R.B.Clark, Oxford, 1964, which discusses the differences between deuterostomes, protostomes, and — wait for it — other creatures with three layers of cells.

Now a possible solution to the mystery is at hand.  Science Nordic has recently regaled us with this delightfully-titled article:

Anus-mouthed worm looks like our earliest ancestor

Which refers to Xenoturbella, a very primitive flatworm-like animal found in deep water off the coasts of Scandinavia.  Its very name suggests that it was long thought to be a Turbellarian, a grouping covering all the mostly non-parasitic flatworms, distinct from the exclusively parasitic flukes (Trematodes) and tapeworms (Cestodes).  However, this group has turned out to be something of a catch-all, and has now been broken up into many.

But Xenoturbella?  As its name suggests, it seems like a rather strange turbellarian.  But recently it was moved to the deuterostomes (on molecular grounds?), even though no-one had actually been able to monitor their development of their eggs until very recently. 

They can be found by taking hundreds of kilos of mud up from the seabed and then waiting for them to crawl out of the mud. But they are so difficult to keep alive that no-one has succeeded in doing this until now.  The Japanese marine biologist Hiroaki Nakano, of Gothenburg University, Sweden, has not only managed to keep them alive – they have even reproduced,”

says Peter Funch of Aarhus University.

They are indeed bilateral, but

it neither has eyes nor a brain – only a hint of a mouth, and the newly-hatched worms have yet to develop an anus.

And when an opening does develop, food goes in and out of the one opening.  So if this little critter is hardly changed over 600 million years, it suggests that the other deuterostomes kept the first orifice as an anus, and when a full alimentary canal evolved, food was taken in through the second.

UPDATE: 2nd July 2018

I have just read this, the Wikipedia article on Xenacoelomorpha (including Xenoturbella)

The clade Xenacoelomorpha, grouping Acoelomorpha and the genus Xenoturbella, was revealed by molecular studies.  Initially it was considered to be a member of the deuterostomes, but a more recent transcriptome analysis concluded that it is the sister group to the Nephrozoa, which includes the protostomes and the deuterostomes, being therefore the basalmost bilaterian clade.

But back to our sea cucumber.  What about its larval development?  That of the related starfish is quite complicated.  Like all echinoderms, it starts as a bilaterally symmetrical dipleurula, and develops through several forms.  Then a radial adult rudiment forms.  Referring again to Living Invertebrates,
The final stages of metamorphosis are completed rather rapidly.  A new mouth breaks through on the larval left side through the middle of the ring canal, while a new anus opens on the larval right side, thus producing an adult axis at right angles to the larval axis.  The 5 radial canals grow out and develop tube feet, the body takes on the adult shape, and the young sea star crawls away to a new life.
This had me somewhat alarmed, so I searched online, without success.  However, in Echinoderms by D Nichols (1962) the larval forms of all the main groups are compared.  The sea urchins follow a path almost a complicated as the starfish, but the sea cucumbers turn out to be somewhat simpler.  The original mouth is retained, but at a certain stage
The blastopore closes off, but the anus opens again a little way from it, a curious but fairly common phenomenon of development in deuterostomes which is not fully understood.
So we reach the end of this little journey, but I have a suggestion for a reply to someone whose utterance is only worthy of being flushed into the sewage system:

Spoken like a true deuterostome!