616 



ARTHROPODA 



SUB-KINGDOM VII 



The normal Trilobite leg has two branches arising from a basal joint, or 

 coxopodite, which is prolonged into a gnathobase. The inner branch, or endo- 

 podite, has normally six joints. The outer branch, or exopodite, has a long 

 proximal joint, with a distal multiarticulate portion. Long setae extend 

 posteriorly, and on the distal portion they are so crowded as to make a con- 

 spicuous fringe, imparting a characteristic appearance to the leg. 



Besides the antennules, the cephalon bears four pairs of pediform biramous 

 appendages, with large gnathobases functioning as manducatory organs. Of 



Trial-Virus Becki, Green, a, Restored thoracic limbs in transverse 

 section of the animal ; 1>, Section across anterior portion of pygidium ; 

 c, Section across posterior portion of pygidium (after Beecher). 



FIG. 1269. 



Triarthrus Becki, Green. 

 Dorsal view of second thoracic 

 leg, with and without setae and 

 without gnathobase. en, Endo- 

 podite ; ex, Exopodite (after 

 Beecher). 



these the first may be correlated with the posterior antennae of higher Crustacea. 

 In structure and function they are true mouth appendages, like the second 



a b 



FIG. 1270. 



Ti-iiiiK-ltus concentricus, Eaton. Utica Slate (Ordovician) ; Rome, New York. A, Left half of pygidium 

 and three thoracic segments, with test removed, and showing fringes of the exopodites. B, Ventral aspect of 

 same, a, Endopodite ; b, Exopodite. 10 /i (after Beecher). 



pair of nauplius limbs. The second pair, corresponding to the mandibles of 

 higher forms, and the third and fourth, corresponding to maxillae, have the 

 same structure as the first, with large gnathobases and fringed exopodites. 

 The thoracic and abdominal limbs are of the same biramous type. The endo- 

 podites are jointed, crawling legs ; posteriorly, especially on the pygidium, 



