Answer
a. protostomes
Work Step by Step
Protostomes are organisms whose mouths originate from the first opening that develops in the embryo. In contrast, deuterostomes have mouths formed from the second opening that developed in the embryo. Many bilateral invertebrates such as flatworms, annelids, mollusks, roundworms, and arthropods are protostomes.
Vertebrates are all deuterostomes. Only animals with bilateral symmetry can be either protostomes and deuterostomes; animals with radial symmetry are neither.
