226 THE ANATOMY OF INVERTEBRATED ANIMALS. 



a shield-shaped labrum, which lies in front of the mouth, has 

 been preserved in some specimens. The body consists of a 

 cephalic shield (Fig. 57, A) ; of a variable number of mov- 

 ably-articulated thoracic somites (Fig. 57, -S) ; and of a py- 

 gidium, composed of a variable number of the somites which 

 succeed the thorax, united together (Fig. 57, C). 



Each thoracic somite presents a median portion, convex 

 from side to side, termed the axis or tergum, and two flat- 

 tened lateral portions, the pleura. The former overlap one 

 another largely when the body is extended, the latter when 

 it is flexed, and the freedom of motion permitted by this ar- 

 rangement is so great that many Trilobites were able to roll 

 themselves up like wood-lice, and are found fossilized in that 

 condition. At the lateral edge of each pleuron, the cuticular 

 substance of which it is composed folds inward, and can be 

 traced on the ventral or sternal side for some distance. But 

 in the middle of the ventral region no indication of a sternum 

 is discoverable. It may, therefore, be concluded that the 

 sternal region of the somite was of a soft and perishable na- 

 ture ; and that the thoracic somite of a Trilobite resembled 

 one of the abdominal somites of a crab in this and in some 

 other respects. 



The glabellum (Fig. 57, 4), or central raised ridge of the 

 cephalic shield, is a continuation of the thoracic axis, the lo- 

 cation of its sides perhaps referring to the number of primi- 

 tive somites it represents. The limb, or lateral area on either 

 side, answers to a thoracic pleuron ; its thickened margin 

 (Fig. 57, 1) is produced into two longer or shorter posterior 

 angles (g) ; interiorly, the marginal band is reflected inward 

 for a short distance, as the subfrontal fold, the remaining 

 sternal area being incomplete. A median movable plate 

 answers to the labrum of Apus and Limulus. On the occip- 

 ital or lateral margin of the limb a suture (Fig. 57, 5) com- 

 mences, and, passing between the eye and the glabellum, 

 meets that of the opposite side either in front of the latter, 

 or on the margin of the limb, or on the subfrontal fold, and 

 is connected with the labral suture by one or two sutures. 

 The limb is thus divided into two parts — one fixed (the fixed 

 gena, Fig. 57, «), attached to the glabellum ; the other sep- 

 arable (the movable gena, Fig. 57, b), on which the eye is 

 placed. The eyes are absent in some genera. In others they 

 occur as isolated ocelli ; or in groups, their interspaces being 

 occupied by the common integument ; or they may resemble 

 the compound eyes of other Arthropods. 



