XI 



PHYLUM ARTHROPODA 



605 



covered by a carapace or cephalic shield (r..s7/), the elevated median region of which, 

 known as the (jlahella (f//), usually presents three or four transverse grooves, 

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

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

 xufiire (./".*). i n t an inner or mesial portion, the fixed cheek (/*.<), continues with 

 the glabella, and an oiiter free portion, the movable check (m.c) ; the latter 

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

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

 the carapace is continued, as in Apus, into a sub-frontal plate (B, *./.), to the 

 posterior edge of which is attached a large labrum or hypostome (/?"'). In many 

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

 angles of the carapace are often produced into spines. 



'"fc. 



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

 head of Phacops fecundus. c.sh. cephalic shield; e. eye ; J.c. fixed cheek ', j'.s. frontal 

 suture; fjl. glabella; Ibr. labrurn : m.c. movable cheek; p. pygidium ; pi. pleura; .././>. 

 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 dow r nwards into spines. 

 The abdomen is covered by a caudal shield OY pygidium (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. 



