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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.
Probably it differed from the existing gastrula only in one essential point, though extinct millions of years ago. Some of the cells of the primary germ-layers probably became ova and others fertilising sperm. We base these hypotheses on the fact that we do to-day find the simplest form of sexual reproduction in some of the living gastraeads and other lower animals, especially the sponges.
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