THE EVOLUTION THEORY 



105 



shield which in Paradoxides is ornamented with long horns. The 

 thorax consists of sixteen to twenty segments. At the posterior 

 end is the tail or pygidium, which in this type is small, but in 

 other Trilobites sometimes of considerable dimensions. As with 

 their modern edible cousins, the crabs and lobsters, the dorsal 

 surface of the body is protected by a strong calcareous crust, 

 whilst the ventral surface, with the legs and gills, is relatively 

 soft. 



It is remarkable that ap- 1 2 



parently all Cambrian Trilo- 

 bites were blind, whereas in 

 the next formation,, the Silu- 

 rian, species are found that 

 have highly developed facetted 

 eyes. This phenomenon may 

 be explained in two ways. 

 The Cambrian Trilobites were 

 either denizens of the deep 

 sea, living in depths which no 

 ray of light can reach, or, what 

 is still more probable, the at- 

 mosphere in those early days 

 of the earth-history contained 

 as yet so much vapour that 

 there was a permanent twi- 

 light which the sunrays could 

 not penetrate. 



Of other animals found in 

 the Cambrian system we must 



notice the Brachiopods, several species of mussels and snails, and 

 finally some low forms of Echinoderms, the Cystoidea. Higher 

 animals are completely absent. The Brachiopoda are only 

 represented by some of the lower forms. They nourish in the 

 subsequent systems there are many species in the Devonian, 

 Carboniferous, and Permian systems and then become gradually 

 extinct. To-day they form an unimportant class with but a 

 few species which live at enormous depths of the sea and are 

 apparently becoming extinct. The Brachiopoda are distinguished 

 from the mussels by having dorsal and ventral, instead of right 



FIG. 25. TRILOBITES. 



(I) Paradoxides bohemicus (Cambrian) ; 



(2) Phacops schlotheimi (Devonian) ; 



(3) The same, coiled up. 



