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Updated: June 23, 2025
They fall in several genera, some of which are characterised by the possession of special polar cells; the body is sometimes roundish, oval, or club-shaped, at other times long and cylindrical. I adhere to the phylogenetically important theory that I advanced in 1876, that we have here real gastraeads, primitive survivors of the common stem-group of all the Metazoa.
These are presented to us when we study the early stages in the development of the embryos of the Metazoa in general.
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.
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