properistoma

The primitive mouth is closed sooner or later in all the vertebrates, and does not evolve into the permanent mouth-aperture; it rather corresponds to the "properistoma," or region of the anus.

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876

The point of origin is always the properistoma, the border of the original primitive mouth of the gastrula, at which the two primary layers pass directly into each other.

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876

All these important modifications take place at first in the fore or head-part of the embryo, and spread backwards from there; here at the hinder end, the region of the primitive mouth, the important border of the mouth (or properistoma) remains for a long time the source of development or the zone of fresh construction, in the further building-up of the organism.

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876