Figure 12.

The trochaea theory. The upper part of the left side shows the ancestral trochaea. The left side shows the life cycle of a trochaea which has added a creeping, benthic stage to its life cycle and established a functional tube-shaped gut by lateral compression of the lateral blastopore lips. The right side shows the life cycle of a protostomian ancestor which has developed a permanent tube-shaped gut by fusion of the lateral blastopore lips and differentiated the archaeotroch into the anterior proto- and metatroch around the mouth and the telotroch around the anus; prototroch and metatroch plus the adoral ciliary zone forms a downstream-collecting ciliary system. From [75].