V GIOANTOSTEAGA 415 



maxillipedes, the most posterior of these being more developed than 

 the rest ; the most anterior is inserted behind the upper lip. The 

 limbs of the trunk and pygidium are biramose ^vith long endo- 

 podite and short exopodite, and with bifurcated epipodial appendages 

 on the basal joint, which may be either filamentous or ribbon-shaped, 

 simple or spirally twisted ; these may safely be assumed to be gills. 

 These gills also seem to occur on the limbs of the pygidium, but in 

 a reduced condition. The enteric canal runs through the body in a 

 straight line to the end of the caudal shield. In front of the mouth, 

 at the lower and anterior edge of the cephalic shield, is found a shell- 

 piece which is called the uppier lip. 



In some of the genera of Trilobites a pretty complete series of successive stages of 

 development (larval stages) has been discovered. In the youngest stages the cephalic 

 shield is present, but the trunk is still quite incomplete. The development of the latter 

 generally occurs in such a way that the pygidium takes precedence of the thorax, 

 and that new segments continually become differentiated at the anterior end of the 

 pygidium. In other words, the thorax differentiates in the order from before back- 

 ward. 



The available material suffices to show : (1) that the Trilobites are Arthropoda, 

 and (2) that they are most nearly related to the Crustacea. Their Crustacean 

 character is unmistakably supported by the fact that the trunk feet are bhamose, 

 and carry epipodial appendages. A closer comparison with distinct orders of 

 Crustacea, however, cannot be carried out, because in the Trilobites no limbs have 

 been found placed in front of the mouth, and comparable with the anterior antennje. 

 ^\Trether these were entirely wanting, or rudimentary, or not capable of petrifaction, 

 is quite uncertain. Should they still be found, then the 5 cephalic limbs could 

 without difficulty be referred to the 5 typical limbs of the Crustacean head, and the 

 Trilobites might then be regarded as original Entomostraca, to be derived from the 

 same racial form as the Phyllojioda. We are prevented from comparing them with 

 the Malacostram on account of the different segmentation of the body and the 

 inconstant number of the segments which does not agree with that of the Mala- 

 costraca. 



The Trilobites were marine. Agnostus (with only 2 thoracic segments), 

 Tnnucleus, Olemis, Paradoxides, Conocephalites, Sao, Oalymenc, Asaplius, Bronteus, 

 Phaeops, Cheirurus, Acidas^iis, Lichas, Proetiis, Harpes. 



II. The Gigantostpaea (Merostomse, Eurypteridse). 



These are also extinct. They lived during the paleozoic epoch. 



Body (Fig. 283). — The elongated scale-covered body of these, the 

 largest of all Arthropoda, falls into head (cephalothorax ?), thorax, and 

 abdomen. The unsegmented head is relatively small, and carries 2 

 compound lateral eyes, and very near the median line 2 ocelli. The 

 thorax and abdomen each consist of 6 segments. The 6th abdominal 

 segment is followed by a caudal stylet or a fin - shaped terminal 

 segment. 



Extremities. — The division described as the head carries 6 pairs 

 of extremities, composed of simple rows of joints, and thus not 



