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In Prophysema the primitive gut is a simple oval cavity, but in the closely related Gastrophysema it is divided into two chambers by a transverse constriction; the hind and smaller chamber above furnishes the sexual products, the anterior one being for digestion. The only material difference between them is that in the sponge the thin two-layered body-wall is pierced by numbers of pores.
In the struggle for life they have found shelter in the body-cavity of other animals. Modern gastraeads. In order to avoid confusion with these, I afterwards gave them the name of Prophysema. The whole mature body of the Prophysema is a simple cylindrical or oval tube, with a two-layered wall. The two strata of cells that form the wall of the tube are the primary germinal layers.
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.
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