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Updated: May 2, 2025
When these are closed they resemble the Physemaria. Possibly the gastraeads that we call Physemaria are only olynthi with the pores closed. Prophysema primordiale, a living gastraead. The same in longitudinal section. Ascula of gastrophysema, attached to the floor of the sea. Olynthus, a very rudimentary sponge. The lowest form of the Cnidaria is also not far removed from the gastraeads.
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.
A corresponding germinal condition, the two-layered gastrula, occurs transitorily in the embryological history of all the other Metazoa, from the lowest Cnidaria and Vermes up to man. From the common stock of the Helminthes, or simple worms, there develop as independent main branches the four separate stems of the Molluscs, Star-fishes, Arthropods, and Vertebrates.
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