XC.—The Malacostraca: Their origin, relationships and Phylogeny

This paper is based on the morphology of certain Cambrian Trilobites. These are much the most primitive Arthropods known, much more so than any Crustacea. Trilobites constitute the most primitive Arthropod Class, and they alone suggest the stages of origin of the Arthropod—the prot-Arthropod with simple eyes and the deut-Arthropod with compound. Both Trilobites and Crustacea are derived from the deut-Arthropod. The organs of Crustacea are derived through those of these stages and from those of the Polychaet; and the head is claimed as the equivalent of the head of the Trilobite and of the prosoma of the Chelicerate without the pre-genital segment. The apparent reduction of segments in the head of Crustacea and other Arthropods is attributed to the reduction of their appendages under cover of the labrum and their incorporation within the alimentary canal. The tagmata and subtagmata and the nomomerism exhibited by the Malacostraca are derived from an ancestral periodicity in the segmentation, which exists still in the Polynoidae among Polychaets, and is exhibited also by the Olenellidae among Cambrian Trilobites. The segmentation of the Malacostraca is claimed also to be the same fundamentally as that of the Scorpions and Eurypterids among the Chelicerates, excepting only that these have lost their analite or telson