PHYLUM ARTHROPODA 



605 



covered by a carapace or cephalic shield (c.sh), the elevated median region of which, 

 known as the glabella («//), usually presents three or four transverse grooves, 

 probably indicating the presence of four or five segments. The lateral regions 

 of tin- carapace are divided by an oblique line of separation, the frontal or facial 

 suture (./".••<), into an inner or mesial portion, thefxed cheek (f.c), continues with 

 the glabella, and an outer free portion, the movable cheek (m.c) ; the latter 

 bears the large paired compound eye (e). In some cases there is an indication 

 of a dona) organ, like that of Apus, on the last cephalic segment. Ventrally 

 the carapace is continued, as in Apus, into a sub-frontal plate (B, s.f.p), to the 

 posterior edge of which is attached a large labrum or hypostome (Ibv). In many 

 Trilobites the hypostome bears a pair of small compound eyes. The posterior 

 angles of the carapace are often produced into spines. 



Fm. 477.— Dalmanites socialis, dorsal aspect; B, the same rolled up; C, under-side of 

 head of Fbacops fecundus. c.sh. cephalic shield ; e. eye ; f.c. fixed cheek ; /.«. frontal 

 suture ; gl. glabella ; Ihr. labrum ; m.r. movable cheek ; p. pygidium ; pi. pleura ; *.f.j>. 

 sub-frontal plate ; th. thorax. (After Gerstaecker.) 



The thorax (th) is composed of a variable number (2-29) of movably articulated 

 segments, which are commonly trilobed, consisting of a median region or axis, 

 and of lateral pleura (pi) often produced backwards and downwards into spines. 

 The abdomen is covered by a caudal shield or pyaidium (p), formed of a variable 

 number of fused segments. Owing to the mobility of the thorax, the Trilobites 

 were able in many cases to roll themselves up like Wood-lice (B). Each of the 

 segments, with the sole exception of the last or anal, bore a pair of appendages. 



The appendages are known only in a few cases. Quite recently a single pair of 

 antenna? (Fig. 478) has been shown to exist in one species, probably attached to 

 the sub-frontal plate. Four pairs of biramous leg-like cephalic appendages 

 have been demonstrated, and the thorax bears slender biramous legs with 

 endo- and exo-podites, and bearing spiral gills. Similar limbs are present on the 

 abdomen. 



