LIFE 



359 



Trilobites were easily the most distinguished forms of Cambrian 

 life. They were not only the highest in organization, but the most 

 characteristic of the period. Their successive genera best distin- 

 guish its successive stages, and their distribution is a chief means of 

 correlating the formations of different regions. Figs. 311 and 323 

 show their three longitudinal lobes, whence their name. Trilobites 

 were kin to the modern crab and crayfish, representatives of the 

 great group Arthropoda (p. 686). They have long been extinct, 

 but the modern horse-shoe crab has some likeness to them. 



Trilobites were well advanced in the scale of development, 

 possessing nearly all the anatomical systems and physiological 

 functions of modern crustaceans. Perhaps their compound eyes 

 are the best index of their development. In this and succeeding 

 periods, the number of eyelets in trilobites' eyes ranged from a 

 score to several thousands. Some of them, however, had no eyes, 

 while others possessed abortive rudiments, implying that their 

 ancestors had possessed them. The acquisition and abortion of so 

 important an organ seem to indicate change in the conditions of 

 life. This may mean no more than migration to deep dark waters, 

 or the habit of burrowing in the mud, where eyes were useless. The 

 eyes of some were raised slightly on crescentic lobes, with the con- 

 vex face outwards (a and c, Fig. 323). In later epochs, these cres- 



f 



Fig. 323. CAMBRIAN CRUSTACEA: a, Holmla (Olcncllus) broggcri Walcott, a 



Walcott, a Cambrian phyllocarid; /, Lcperditia dernwioides Walcott, a Cambrian 

 ostracode. 



