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Updated: June 19, 2025
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.
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.
The same applies to the greater part of the Vermalia or worm-like animals, the various classes and orders of which differ so much in structure. The isolated groups of this rich stem are living branches of a huge tree, the greater part of which has long been dead, and we have no fossil evidence as to its earlier form.
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.
We can say, for instance, that we have inherited the oldest organs of the body, the external skin and the internal coat of the alimentary system, from the Gastraeads; the nervous and muscular systems from the Platodes; the vascular system, the body-cavity, and the blood from the Vermalia; the chorda and the branchial gut from the Prochordonia; the articulation of the body from the Acrania; the primitive skull and the higher sense-organs from the Cyclostomes; the limbs and jaws from the Selachii; the five-toed foot from the Amphibia; the palate from the Reptiles; the hairy coat, the mammary glands, and the external sexual organs from the Pro-mammals.
The great majority of the Vermalia have these three features, and they are all wanting in the Platodes; in the rest of the worms at least one or two of them are developed. Zoologists differ as to their position in classification. Otherwise the organisation of the two classes is the same. These are the Nemertina and the Enteropneusta.
While we seek the most important of these palingenetic forms among the groups of Coelenteria and Vermalia, it is understood that not a single one of them must be regarded as an unchanged, or even little changed, copy of the extinct stem-form.
As we saw there, the unarticulated Tunicates and the articulated Vertebrates must be regarded as two independent stems, that have developed in divergent directions. But the common root of the two stems, the extinct group of the Prochordonia, must be sought in the vermalia stem; and of all the living Vermalia those we have considered give us the safest clue to their origin.
Coelenteria, without anus and body-cavity. : 6. Gastraeades. With two germ-layers. : 6. Gastrula. Hydra, Olynthus, Gastremaria. 6 to 8. Coelenteria, without anus and body-cavity. : 7. Cryptocoela. Convoluta, Proporus. 6 to 8. Coelenteria, without anus and body-cavity. : 8. Rhabdocoela. Vortex, Monotus. 9 to 11. Vermalia, with anus and body-cavity. : 9. Provermalia. Gastrotricha.
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