414 



COMPARATIVE ANATOMY 



CHAP. 



FiFSt Appendage to the Class of Crustacea. 

 The Trilobites, GlgantostFaea, Hemiaspidse and Xiphosura. 



I. The Trilobites. 



These extinct Articulata are found only as fossils, and only in 

 palaeozoic formations. 



Body (Fig. 281). The integument of the upper side was hard; 

 but on the under side soft. The body falls into 3 divisions, 

 cephalic shield, thorax, and caudal shield (pygidium). Each of these 

 divisions is again divided by 2 almost parallel longitudinal dorsal 

 furrows (the thorax most distinctly) into an arched middle area 



Fig. 282.— Restored trunk segment of a Trilobite, 

 transverse section (after Walcott). ? , Ehacliis ; jy, 

 pleura ; ep, epipoclial appendages ; en, endopodite ; 

 f.'', exopodite ; d, intestine. 



Fig. 281.— Cheirurus Quenstedtil, 

 dorsal view. 



(rhachis) and 2 lateral areas (pleura). The cephalic shield is 

 unsegmented, semicircular, or crescent-shaped, with the rounded part 

 to the front; it generally carries 2 large compound eyes. The 

 occurrence of simple eyes is very doubtful. The thorax consists of a 

 varying (usually rather large) number of freely moving segments. 

 The caudal shield seems composed of a varying number of segments 

 more or less completely fused together. Nearly all (or all ?) Trilobites 

 were able to roll up their bodies like woodlice, so that the anterior 

 edge of the cephalic shield and the posterior edge of the caudal shield 

 touched one another. 



Limbs (Fig. 282). — These are retained only in rare cases. They 

 are slender and long, and more or less like one another, segmentally 

 repeated from the cephalic shield to the end of the caudal shield. 

 Under the cephalic shield there are 4 pairs of limbs reckoned as 



