Branchiostomidae (in which the trabecular arch is undifferentiated), is readily explained.    From Wordnik.com.  [Life and Letters of Thomas Henry Huxley — Volume 3] Reference

And, behold! there is a lengthy attempt to write the matter up in full, in which, among other things, he was seeking to show that, on this basis, the mode of termination of the notochord in the Craniata, and in the Branchiostomidae (in which the trabecular arch is undifferentiated), is readily explained.    From Wordnik.com.  [The Life and Letters of Thomas Henry Huxley]