THE CAMBRIAN PERIOD 



4 II 



projection), a group of 



animals belonging to the 



same phylum as the 



crabs and lobsters. The 



name trilobite is a very 



descriptive one, since the 



animal was marked by 



two grooves running 



lengthwise of the body, 



which divided it into 



three, usually well- 

 marked lobes. Trans- 

 versely, there were also 



three divisions : the head 



shield (cephalon) ; the 



body, composed of jointed 



segments (thorax) ; and 



the tail shield (pygidium). 



Trilobites were marine 



animals and had delicate 



antennae, doubtless for 



touch, and numerous legs 



and breathing organs 



(Fig. 379). They were 



probably able both to 



walk and swim. Their 



trails and burrows show 



that they burrowed and pushed their way through the muds and soft 



sands. A series of tracks probably 

 made by a trilobite is shown in 

 Figure 380. 



The eyes of trilobites were usually 

 raised, crescent-shaped elevations 

 and were compound like those of an 

 insect (Fig. 381), the number of 

 lenses in each eye varying in differ- 

 ent species from 14,000 to 15,000. 

 Fig. 381.— Eye of a Devonian A few spec i es we re eyeless. Cam- 



trilobite, much enlarged, snowing the . mi* /t** r> a r> r> tw 



many eyelets forming the compound bnan trilobites (Fig. 382 A, B, C, D) 



eye. (N. Y. Geol. Surv.) varied greatly in size, in form, and in 



Fig. 380. 



Tracks supposed to have been made by 

 a trilobite. 



£ 



h. 



