NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS II7 



to the labrum from the frontal region of the carapace. As in Lvmulus 

 again, the chehcerae have only dorsal muscles, which arise on the 

 anterior part of the carapace (fig. 49 D, Chlmcls). 



The ancestors of the modern Mandibulata were represented in the 

 more generalized members of the Protarthropoda that persisted after 

 the trilobite-chelicerate branch had been given ofif from the main stem 

 (fig. 54). The Protomandibulata undoubtedly retained the primitive 

 centipedelike form of the protarthropods, but, as shown in the embry- 

 ology of modern Mandibulata, the head at this stage must have been 

 a composite protocephalon (fig. 39 C, Prtc) formed by an intimate 

 union of the first somite (/) with the highly developed prostomial 

 acron {Acr). It carried, therefore, the eyes (£), the labrum (Lni), 

 the acronal appendages, or first antennae (lAnt), and the appendages 

 of the included somite, which became a second pair of antennal organs 

 {2 Ant). The distinctive feature of the early mandibulates, however, 

 was the presence of a pair of jaws, the mandibles (Md), developed 

 from the bases of the appendages of the first postcephalic somite. 

 Probably also the appendages of the following two somites were 

 reduced in size and modified in a manner suggestive of their future 

 transformation into maxillae ; and perhaps a pair of paragnathal lobes 

 was developed between the mandibles and the first maxillary appen- 

 dages, since these structures are not present in the chelicerate branch. 



The Crustacea represent the first offshoot from the mandibulate 

 section of the arthropod stem that has given rise to a specialized group 

 of modern forms (fig. 54). The wide recurrence among the Crustacea 

 of cursorial appendages identical in segmentation with the legs of the 

 trilobites can leave little doubt that the primitive crustaceans were 

 polypodous walking animals, living on the bottom of the water or on 

 aquatic plants along the ocean shores, and adapted to life in the water, 

 as were the trilobites, by the development of branchial organs on exite 

 lobes of the coxopodites. According to this view, the natatory appen- 

 dages of swimming or purely pelagic Crustacea are legs that have 

 been modified secondarily for swimming purposes, just as the gnathal 

 appendages have been modified for feeding. It is a sound principle 

 of ecology that pelagic forms in all cases have been derived from 

 benthonic forms (see Hesse, Allee, and Schmidt, 1937, p. 179), and 

 the fact that many of the more generalized modern Crustacea are 

 pelagic is no argument that such forms are ancestral. The frequent 

 biramous structure of crustacean appendages is entirely a crustacean 

 feature, since the exopodite is a specially developed outer branch of 

 the basipodite, and therefore has no counterpart in the Trilobita or 

 in any other arthropod group. 



