300 CKUSTACEA. TRILOBITA 



The appendages of the thorax are long, but gradually 

 decrease in size backwards, and consist of a coxopodite 

 (fig. 128 C, c) bearing the endopodite (a) and the exopo- 

 dite (b) which are of nearly equal length. The endopodite 

 is formed of six joints, and probably served as a swimming 

 organ. The exopodite consists of a long basal joint fol- 

 lowed by a part consisting of numerous short joints ; it 

 bears setae along its posterior edge and was probably 

 adapted for crawling. The inner prolongations of the 

 coxopodites served as jaws. The limbs in each pair are 

 widely separated, and in each segment the ventral cuticle 

 between their bases is strengthened by a median longi- 

 tudinal ridge and one or two oblique ridges on each side. 

 On the posterior part of the thorax some of the joints of 

 the endopodites become flattened. 



The appendages of the pygidium are similar to those 

 on the posterior part of the thorax, but are more distinctly 

 leaf-like owing to the flattening and expansion of the first 

 segments of the endopodite which bear setae ; the exopo- 

 dite is slender. The anal opening is on the last segment 

 (or telson) near the end of the pygidium. 



In some fine-grained deposits, especially in the Lower 

 Palaeozoic rocks of Bohemia, the larval forms of Trilobites 

 are found well preserved, and by obtaining specimens of 

 different ages it is possible to trace out the changes which 

 occurred in the development of the individual. In the 

 earliest stage (fig. 129 A), called the protaspis by Beecher, 

 the body is discoid or ovate in form, and consists of a large 

 cephalic region and a small pygidial part ; the axis is 

 distinct, and is marked by furrows ; eyes, when present, 

 are at or near the outer front margins of the shield (fig. 

 129 E), and the free cheek, if visible on the dorsal surface, 

 is narrow (G). The glabella usually reaches the front 



