19. Which of the following would rank lowest in cephalization? A. Cnidarians B. Arthropods C. Mammals D. Mollusks

It is cnidarians, cnidarian phylum has all diploblastic animals and they exist in form of polyp (stationary form) or medusa (free swimming form). Diploblastic animals only have two cell layers, they do have mouth but the body division of head and trunk does not exist in this phylum. Hydra is a classic example of cnidarians. While they have anterior and posterior end, yet no clearly defined cephalic structure and therefore, they would rank lowest in cephalization