Even where a large number of the somites have fused, there is generally a marked change in the character of the appendages after the fifth pair, and since the integumental fold which forms the carapace seems to originate from this point, it is usual to take the fifth somite as the morphological limit of the cephalon throughout the class.
The three pairs of appendages present in the " nauplius " larva show certain peculiarities of structure and development which seem to place them in a different category from the other limbs, and there is some ground for regarding the three corresponding somites as constituting a " primary cephalon."
This is especially the case in the anterior part of the body, where, in correlation with the " adaptational shifting of the oral aperture " (see Arthropoda), a varying number of somites unite to form the "cephalon " or head.