The stucly of the arthropod head offers some of the most interesting and important evidence indicating the paths of development followed in the evolution of the various types of arthropods, but the evidence from this source has not been brought together, and nrost of the important types of head capsule have not been figured. The present paper, and the one following it, are offered with a view to supplying this needed information; and in these papers especial attention has been paid to the Crustacea, since these furnish the proto-types of many of the modifications met with in the Insecta, Chilopoda, Diplopoda, etc., and the carcinologists have neglected to furnish us with figures of these important forms.