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Updated: May 14, 2025
Naturally, I do not mean that the Vertebrates have descended from the Tunicates, but that the two groups have sprung from a common root. The chordaea-theory received the most valuable and competent support from Carl Gegenbaur.
This is the stem of the Tunicates. One member of this group, the sea-squirt, very closely approaches the lowest vertebrate, the Amphioxus, in its essential internal structure and embryonic development.
When we try to construct an animal frame of the simplest conceivable type, that has some such primitive alimentary canal and the two primary layers constituting its wall, we inevitably come to the very remarkable embryonic form of the gastrula, which we have found with extraordinary persistence throughout the whole range of animals, with the exception of the unicellulars in the Sponges, Cnidaria, Platodes, Vermalia, Molluscs, Articulates, Echinoderms, Tunicates, and Vertebrates.
It has descended from the Cyclostoma by a profound degeneration, and these in turn from the fishes; even the Ascidia and the whole of the Tunicates are merely degenerate fishes! Following out this curious theory, Dohrn came to contest the general belief that the Coelenterata and Worms are "lower animals"; he even declared that the unicellular Protozoa were degenerate Coelenterata.
The heart contracts first from the rear to the front, stands still for a minute, and then begins to beat the opposite way, now driving the blood from front to rear; the two large vessels that start from either end of the heart act alternately as arteries and veins. This feature is found in the Tunicates alone.
In the alimentary apparatus there are the thymus-gland and the thyroid gland, the seat of goitre and the relic of a ciliated groove that the Tunicates and Acrania still have in the gill-pannier; there is also the vermiform appendix to the caecum.
The tunicates are the only class of animals that have a real cellulose or woody coat. Sometimes the cellulose mantle is brightly coloured, at other times colourless. Not infrequently it is set with needles or hairs, like a cactus. Often we find a mass of foreign bodies stone, sand, fragments of mussel-shells, etc. worked into the mantle. This has earned for the Ascidia the name of "the microcosm."
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.
The most remarkable case of its appearance among higher forms is that of the marine Ascidians, or tunicates close allies of the true vertebrates where reproduction by budding and the formation of wonderfully elaborate star-like forms produced by budding and the cohesion of the budded individuals as one composite individual are well known.
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.
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