ARTHROPODA — CRUSTACEA —TRILOBITES 291 



The skeleton (Fig. 1 26) consists of a thick dorsal shield, a very 

 thin ventral membrane, and the covering of the body appendages. 

 The dorsal shield is divided into head (cephalon), thorax and 

 pygidium. The headjhield comprises three pieces, — a median 

 one consisting of the glabella and fixed cheeks, and the two free 

 cheeks, the three united by thin chitinous joints. The free 

 cheeks may represent the pleurae of one of the anterior head 

 segments. The thoracic shield consists of many segments, 

 — transverse divisions, varying from two in Agnostus to twenty- 

 nine in Harpes, all united by thin chitinous joints ; each of these 

 segments is made up of a central axial lobe and two pleural 

 lobes firmly united into one piece so that movement takes 

 place only between the segments. The pygidium consists of 

 a single piece, due to the welding of a number of similar seg- 

 ments. 



On the ventral surface of the head are five pairs of appendages, 

 all of which, except the anterior pair, are similar in appearance 

 and function, differing thus from those of Apus (p. 300) and 

 the crayfish (p. 275). 



Muscles. — The bodies of many trilobites, as Calymene, 

 possessed the power of enrollment, like the pill-bug among the 

 isopod crustaceans at present. This was brought about by the 

 contraction of the ventral muscles (p. 278). By this means the 

 soft under surface was protected and the animal exposed to 

 attack only along the hard dorsal shield. 



Sight. — The organs of sight vary greatly among trilobites ; 

 when present they are situated on the free cheeks (except in 

 Harpes, where they are on the fixed cheeks). A few genera, 

 as Agnostus, and the larvae of many forms appear to have 

 been totally bUnd ; others, as Conocoryphe, have small simple 

 eyes, granule-like, at the end of a raised line {eye-line, Fig. 126, F) 

 running from the anterior end of the glabella outward to the free 

 cheeks. The majority of trilobites, however, had compound eyes 

 of varying size ; these either have (i) the separate facets covered 

 by a continuous smooth or granular, horn-like, protective layer 



