A zoologist finds a mysterious worm-like animal, and is trying to identify it. What characteristic would he need to find in the embryo to be able to classify it as a protostome?

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.

an adaptation is a change or process by which an organism takes to be better suited for its environment. for example; migration is a behavioral adaptation, this is needed because birds are not built to last extremely cold climates. there are different adaptations because there are different species needing to adjust to different things, bears don't migrate; they hibernate.