The Branchiopoda have a very variable number of body-segments, with or without a shield, simple or bivalved, and some of the postoral appendages normally branchial.
Ment and of the struc ture of the disk to their identification of "trochus" and "cingulum" with the preoral and postoral wreaths of the trochophore larva.
The antennae (second antennae) are of special interest on account of the clear evidence that, although preoral in position in all adult Crustacea, they were originally postoral appendages.
They are farthest forward in the case of the female organs of the Cirripedia, where the openings are on the first thoracic (fourth postoral) somite.
The larva is provided with postoral and perianal ciliated bands, and on either side with a bunch of long provisional setae.
Lankester (2) was the first to suggest that (as is actually the fact in the Nauplius larva of the Crustacea) the prae-oral somites or prosthomeres and their appendages were ancestrally postoral, but have become prae-oral " by adaptational shifting of the oral aperture."