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We must grant, however, that in the whole stem-history of the Vertebrates the long stretch from the Gastraeads and Platodes up to the oldest Chordonia remains by far the most obscure section. We might frame another hypothesis to raise the difficulty namely, that there was a long series of very different and totally extinct forms between the Gastraea and the Chordaea.

In view of its sound establishment and its profound significance, it may very well claim to be a THEORY, and so should be described as the chordonia or chordaea theory. I first advanced this theory in a series of university lectures in 1867, from which the History of Creation was composed.

The brain is phylogenetically older than the spinal cord, as the trunk was not developed until after the head. Probably this upper pharyngeal ganglion of the lower worms is the structure from which the complex central marrow of the higher animals has been evolved. The medullary tube of the Chordonia has been formed by the lengthening of the vertical brain on the dorsal side.

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.

We can infer from this the common descent of the three groups with all the more confidence when we find the Balanoglossus approaching the Chordonia in other respects. Thus, for instance, the chief part of the central nervous system is a long dorsal neural string that runs above the gut and corresponds to the medullary tube of the Chordonia.

Nevertheless, some of the surviving groups are very instructive, and give us clear indications of the way in which the Chordonia were developed from the Vermalia, and these from the Coelenteria.

Thus the chief advantage in organisation by which the earliest Vertebrates took precedence of the unsegmented Chordonia consisted in the development of internal segmentation. The whole vertebrate stem divides first into the two chief sections of Acrania and Craniota. The Craniota descend directly from the Acrania, and these from the primitive Chordonia.

Here we have the first appearance of the division of the alimentary tube into two sections that characterises the Chordonia. The differentiation of these two parts of the gut in the Enteropneust is just the same as in all the Tunicates and Vertebrates.

It is the same with the human mouth and that of the Craniotes generally. The secondary formation of the mouth in the Chordonia is probably connected with the development of the gill-clefts which are formed in the gut-wall immediately behind the mouth. In this way the anterior section of the gut is converted into a respiratory organ.

But if we exclude the Platodes and the Annelids from this stem, we find a fairly satisfactory unity of organisation in the remaining classes. Among these worms we find some important forms that show considerable advance in organisation from the platode to the chordonia stage.