590 



ZOOLOGY 



SECT. 



whence the trilobation generally characteristic of the group. The head is 

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

 which, known as the gldbetta (gl), 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 suture (f.s.), into an 

 inner or mesial portion, the 

 fixed cheek (f.c), continuous 

 with the glabella, and an 

 outer free portion, the mov- 

 able cheek (m.c) ; the latter 

 bears the large paired com- 

 pound eye (e). In some cases 

 there is an indication of a 

 dorsal organ, like that of 

 Apus, on the last cephalic 

 segment. Ventrally the cara- 

 pace is continued, as in 

 Apus, into a sub-frontal 

 plate (C, s.f.p), to the pos- 

 terior edge of which is 

 attached a large labrum or 

 hypostome (Ibv). In many 

 Trilobites the hypostome 

 bears a pair of small com- 

 pound eyes. The posterior 

 angles of the carapace are 

 often produced into spines. 



The thorax (th) is com- 

 posed of a variable number 

 (2-29) of movably articu- 

 lated segments, which are 

 commonly trilobed, consist- 

 ing of a median region or 

 axis, and of lateral pleura 

 (pi) often produced back- 

 wards and downwards into 

 spines. The abdomen is 

 covered by a caudal shield or 

 pi/gidium (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 appen- 

 dages. 



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

 pair of antennae (Fig. 485) has been shown to exist in one species, probably 

 attached to the sub-frontal plate. There are no true jaws. Four pairs of 

 biramous leg-like cephalic appendages have been demonstrated, and the 

 thorax bears slender biramous legs with endo- and exopodites, and bearing 

 spiral gills. Similar limbs are present on the abdomen. 



FIG. 485. Triarthrus becki, x 2i. A, ventral 

 surface with appendages ; Ep, metasfome ; Hi/, hypo- 

 stome. B, second thoracic appendage, en. endo- 

 podite ; ex. exopodite x 12. (From the Cambridge 

 Natural History, after. Beecher.) 



