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As we find this highly characteristic differentiation of the gut into two different sections in all the Vertebrates and all the Tunicates, we may conclude that it was also found in their common ancestors, the Prochordonia especially as even the Enteropneusts have it. But the number presently increases in the former. In the Craniotes, however, it decreases still further.
The agreement in the structure of the branchial gut of the Enteropneusts, Tunicates, and Vertebrates was first recognised by Gegenbaur ; it is the more significant as at first we find only a couple of gill-clefts in the young animals of all three groups; the number gradually increases.
Bateson believes he has detected a rudimentary chorda between the two. Of all extant invertebrate animals the Enteropneusts come nearest to the Chordonia in virtue of these peculiar characters; hence we may regard them as the survivors of the ancient gut-breathing Vermalia from which the Chordonia also have descended.