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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.
We have to consider the Platodes first, because they are on the border between the two principal groups of the Metazoa, the Coelenteria and the Coelomaria.
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.
In all these rudimentary living coelenteria the sexual cells of both kinds ova and sperm cells are formed by the same individual; it is possible that the oldest gastraeads were hermaphroditic. The sexual cells originally proceeded from the edge of the primitive mouth of the gastraead.
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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