106 LECTUEES ON BIOLOGY 



and left shell-valves. Further, most species have in the interior 

 of the dorsal valve a limy ribbon of very varied shape, which 

 forms the supporting organ of the fleshy mouth-arms. 



A similar poverty and primitive character is exhibited by the 

 Cambrian flora, which so far as we know consists exclusively of 

 simple marine Algae. 



In the second stratum, the Silurian system, there is a far 

 greater variety and a higher development of organisms. But 

 here again Trilobites are the principal species, forming, when 

 contrasted with their Cambrian progenitors, a veritable classic 

 example of progressing development and adaptation to changed 

 conditions of life. We saw already that Trilobites of the Silurian 

 Period had well-developed eyes, whilst the Cambrian species were 

 blind. But it is more surprising that they have now acquired 

 the faculty of curling up, precisely as is done by one of the little 

 terrestrial crustaceans of the family of wood-lice (Armadillidium 

 vulgare) (fig. 25, 2 and 3 ). 



This acquisition is capable of a simple explanation. As Trilo- 

 bites were in the Cambrian system the only large animals there 

 was no need to fear hostile attacks, and consequently no need 

 of special protective measures. But it is different in the Silurian 

 and Devonian strata. Here we find other large animals, mighty 

 Cephalopoda, near relations of the modern Nautilus and the 

 cuttle-fishes, and like them dangerous and voracious robbers. 

 There is no longer peace and safety. An active defence is 

 impossible against these animals which are often more than 

 two metres long, for the soft-skinned ventral surface of the crabs 

 offers a favourable and dangerous point of attack. The faculty 

 of rolling themselves up now becomes an effective protection. 

 When a rapacious enemy came near in search of a toothsome 

 meal the Trilobites simply folded together the cephalic shield and 

 pygidium (fig. 26), and were thus enclosed in a solid armour which 

 offered effective resistance to their enemy's teeth. 



Besides the Trilobites we find here higher crustaceans, 

 enormous Eurypterides, often more than one metre long. But 

 most important of all, we find here and in subsequent systems 

 the Cephalopoda which show in the structure of their shells a 

 gradually increasing complexity. 



Of special interest are the first representatives of the verte- 



