Zoology 10th Edition by Stephen Miller,John Harley

Exercise 16

Members of the class Cephalopoda are the most complex molluscs. In most cephalopods, the shell is reduced or absent, and the mantle is modified for use in jet propulsion. They are fast-moving predators with complex circulatory, nervous, and sensory systems that promote their predatory lifestyles. Reproduction involves the transfer of sperm in spermatophores from males to females. Development is external within egg membranes.
What anatomical feature(s) typical of most molluscs and cephalopod ancestors are absent or highly modified in modern cephalopods, and what accounts for differences in members of this class?