Members of the phylum Cnidaria, which include organisms like jellyfish, corals, and sea anemones, are neither protostomes nor deuterostomes. They represent a separate branch of the animal kingdom known as radiata. This group diverged before the evolution of the protostome-deuterostome distinction.
In traditional classifications, protostomes are characterized by a developmental sequence where the mouth forms before the anus during embryogenesis, while in deuterostomes, the anus develops before the mouth. Cnidarians, however, display a simpler body plan and developmental patterns that do not fit neatly into either category.
Their features include a dual tissue layer and the presence of specialized cells called cnidocytes that contain nematocysts, which are used for capturing prey. This unique biology highlights the evolutionary diversity that exists among animals and stresses the importance of understanding these distinctions in developmental biology.