Final answer:
The most varied morphological trait among all protostomes is the type of body cavity.
Step-by-step explanation:
The most varied morphological trait among all protostomes is the type of body cavity. Protostomes are classified as coelomates, meaning they have a true body cavity called a coelom that is lined with mesoderm. However, the type of body cavity can differ among protostomes. Some protostomes have a coelom formed through a process called schizocoely, where specific blastomeres migrate and form clumps of mesodermal tissue that expand and merge to form the coelom. Others have a coelom formed through a process called enterocoely, where pouches of mesoderm pinch off from the endoderm and fuse to form the coelom.