44 THE STORY OF THE EARTH AND MAN. 



of tlieir aflfinities has long been an unsettled one 

 with zoologists. Hundreds of species are known, 

 some almost microscopic in size, others a foot in 

 length. All are provided with a broad flat horseshoe- 

 shaped head-plate, which, judging from its form and 

 a comparison with the modern king-crabs or horse- 

 shoe-crabs, must have been intended as a sort of 

 mud-plough to enable them to excavate burrows or 

 hide themselves in the slimy ooze of the ocean bed. 

 On the sides of this buckler are placed the prominent 

 eyes, furnished with many separate lenses, on pre- 

 cisely the same plan with those of modern crustaceans 

 and insects, and testifying, as Buckland long ago 

 pointed out, to the identity of the action of light in 

 the ancient and the modern seas. The body was 

 composed of numerous segments, each divided trans- 

 versely into three lobes, whence they have received 

 the name of Trilohites, and the whole articulated, so 

 that the creature could roll itself into a ball, like the 

 modern slaters or wood-lice, which are not very dis- 

 tant relatives of these old crustaceans.* The limbs 

 of Trilobites were long unknown, and it was even 

 doubted whether they had any ; but recent discoveries 

 have shown that they had small jointed limbs useful 

 both for swimming and creeping. The Trilobites, 

 under many specific and generic forms, range from 



* "Woodward has recently suggested affinities of Trilobites 

 with the Isopods or equal-footed crustaceans, on the evidence 

 of a remarkable specimen with remains of feet described by 

 Billings. 



