Which statement best describes protostomes?

The statement that best describes protostomes is that they exhibit spiral and determinate cleavage and that the blastopore becomes the mouth. In protostomes, during embryonic development, the first cleavage occurs in a spiral pattern, meaning that the cells are arranged at a slight angle to the previous layer of cells. This contrasts with deuterostomes, where cleavage is radial and more symmetrical.

Additionally, the term "determinate" refers to the fact that the fate of the cells is determined early in development; that is, each cell has a specific function that it will ultimately fulfill. In contrast, in deuterostomes, the cells are considered indeterminate, which allows for greater potential for cell fate flexibility.

The development of the blastopore into the mouth is another key characteristic of protostomes. The blastopore is the first opening that forms during the early stages of embryonic development, and in protostomes, it becomes the mouth, while the anus develops later from a second opening.

This combination of these developmental features—spiral and determinate cleavage along with the blastopore becoming the mouth—is a defining characteristic that distinguishes protostomes from other groups, such as deuterostomes where these traits are different.