Work Step by Step
Option A is incorrect. The coelom is the body cavity that is lined with mesoderm. Option B is incorrect. The protostomes are the group of animals in which the first opening in the embryo later becomes the mouth of the animal. Option C is incorrect. Deuterostomes are the group of animals in which the first opening in the embryo becomes the anus and a secondary opening becomes the mouth. Option D is correct. The blastula develops after a number of cell divisions in the fertilized egg, or zygote.