SECT. XIII THE TRILOBITES 221 



of a very natural action which we may safely assume 

 went hand in hand with the development of the 

 primitive head-shield already described. At the 

 approach of an enemy the forehead would be pressed 

 against the ground, the thorns, if there were any on 

 the posterior dorsal fold of the fifth segment, would be 

 somewhat erected by the bending under of the head 

 or humping of the back. In such simple movements 

 we have the first step towards rolling up. 



This method of defence by rolling up is one of 

 considerable biological interest (see Fig. 54). In one way 

 it is a very perfect method of defence, but in another 

 it is very fatal. Its perfection is clear from the periods 

 of geological time through which the Trilobites lived ; 

 its fatality in the fact that it admits of no further 

 development. Hence the Trilobites, at least all which 

 failed to develop shells, have died out, as unable to 

 protect themselves from new and more powerful 

 enemies, or from old enemies when these latter had 

 once learned to overcome this method of defence- 

 The development of shell folds, which, except in the 

 case of bivalve shells, are clearly inconsistent with the 

 habit of rolling up, render it unnecessary. They 

 make it possible to develop new and more plastic 

 methods of defence, to which we owe the preservation 

 and the rich and varied development of the whole 

 class of modern Crustacea. 



TJie Trilobite Liinbs.--\\i spite of the great progress 

 which has been made in our knowledge of the limbs of 



