Subphylum Trilobita. (C & V fig. 7.106)                                       

The primitive arthropod body plan is thought to have consisted of an elongate multi-segmented body with each segment bearing a pair of jointed appendages. No such animals exist today, but the fossil record contains a number of groups with this plan. One of the more familiar is the Subphylum Trilobita, which were abundant and widely distributed in Paleozoic seas. The trilobite body was flattened, oval and divided into three parts. They had an anterior head of 4 fused segments bearing antennae and compound eyes followed by a large number of segments, each bearing a pair of jointed, two-branched (biramous) appendages. Study the trilobites on display. The biramous legs had a dual purpose: The upper feathered branches were gills and swimming paddles; the lower branches were for walking. The short anterior legs probably aided in feeding.

A major trend within the arthropods is toward increased specialization of the jointed appendages for functions other than locomotion, and along with this specialization of groups of body segments into body regions. As you examine characteristics of the major groups of arthropods keep these trends in mind. Note different modifications of the jointed appendages for life in different environments. Note modifications to the outer exoskeleton of each group.