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When we turn from the Tunicates to the closely-related Amphioxus we are astonished at first to find an apparent retrogression in the formation of the vascular system. A number of branchial vascular arches, which effect respiration and rise in the wall of the branchial gut from belly to back, absorb oxygen from the water and give off carbonic acid; they connect the ventral with the dorsal vessel.
As both the Tunicates and the Vertebrates develop from the same chordula, we may infer that there was a corresponding common ancestor of both stems. We may call this the Chordaea, and the corresponding stem-group the Prochordonia or Prochordata. We shall see presently how this conclusion is justified in the present condition of morphological science.
If we come now to consider the more partial phosphorescence of the sea, we find that it is due to animals belonging to almost every group of marine forms to Echinoderms, or creatures of the sea-urchin and star-fish type, to Annelid worm, to Medusidæ, or jelly-fish, as they are popularly called, including the "great domes" and the "silvery disks" of the passage above quoted from Professor Martin Duncan, to Tunicates, among which is the Pyrosoma, to Mollusks, Crustaceans, and in very many cases to Actinozoa, or forms belonging to the type of the sea anemone and the coral polyp.
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.
Belonging as they undoubtedly do to the order of the Tunicates, their exquisitely appropriate and elegant costume may be safely allowed to speak for itself.
With more or less modification in detail, the embryo has been observed to pass through these successive evolutional stages in sundry Sponges, Coelenterates, Worms, Echinoderms, Tunicates, Arthropods, Mollusks, and Vertebrates; and there are valid reasons for the belief that all animals of higher organisation than the Protozoa, agree in the general character of the early stages of their individual evolution.
In these important Vermalia the foremost section of the gut has already been converted into a gill-crate, and the vascular arches that rise in the wall of this from the ventral to the dorsal vessel have become branchial vessels. We have a further important advance in the Tunicates, which we have recognised as the nearest blood-relatives of our early vertebrate ancestors.
These instructive Copelata, comparable to permanent Ascidia-larvae, come next to the extinct Prochordonia, those ancient worms which we must regard as the common ancestors of the Tunicates and Vertebrates.
In the first edition of this work I endeavoured to prove, on the strength of Kowalevsky's epoch-making discoveries, that "of all the animals known to us the Tunicates are undoubtedly the nearest blood-relatives of the Vertebrates; they are the most closely related to the Vermalia, from which the Vertebrates have been evolved.
The discovery was confirmed by other zoologists, and there can no longer be any doubt that of all the classes of invertebrates that of the Tunicates is most closely related to the vertebrates, and of the Tunicates the nearest are the Ascidiae.
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