Protostoma

It is frequently stated by critics of the protostoma hypothesis that it involves a reversal of dorsal and ventral sides during the evolution of Vertebrates from their invertebrate ancestors but it will be gathered from what has been said that this criticism rests on a misunderstanding.

The opening of this, the protostoma, becomes elongated, taking on an elliptical shape, and then it narrows in the middle, its outline becoming that of a dumb-bell.

The protostoma with its wall—the protorhabdion—is the major part of the stoma.

Conventionally, the stoma is divided into three parts: the anterior chamber or cheilostoma, the middle or protostoma, and the posterior or telostoma.