to classify it as a protostome, zoologist must see for the mouth development in the embryo.
Explanation:
The protostome are the animals which are unique to their three germ layer in embryonic development, with the bilateral symmetry as they have the ability to fly with other protostome developments.
the one characteristic feature to distinguish them as the protostome, mouth part development is to be sought as the mouth develops but not the anus in the embryo. the mouth parts serves unique to the group and their feeding structures.
I think it's aeration but I'm not 100% sure though.
If the earth didn't rotate, currents would flow in straight lines from pole to equator. :)