It is true that the protostome developmental sequence arose just once in evolutionary history.
Step-by-step explanation:
During evolution the Bilateria had been into two major divisions. One was the protostomes and another was the deuterostomes. It is all categorized during the embryonic development.
It is differentiated by the formation of anus and mouth. The protostomes include the annelids, molluscs and arthropods. Studies reveal that Protostomes are derived from their prebilaterian ancestors.
The protostome developmental sequence has been aroused only once during evolution which lead to the evolution of more than thousands of new species. They then were subdivided into two major group, namely Lophotrochozoa and Ecdysozoa.