INVEETEBRATE ANIMALS. 63 



Group 162.— Order TRILOBITA. rpftg, three ; XofShg, a lobe. 

 The species of this order are known only as Palaeozoic 

 fossils. The eyes when present are compound, but no 

 antennae or gills have been detected, and very little is 

 known of their locomotive organs. Burmeister thought 

 that the Trilobites swam, back downwards, in shoals near 

 the coast, rolling themselves into a ball as a defence 

 when attacked. Dr. Buckland compared them with 

 Serolis (see Group 164). 



Group 163.— Order MEEOSTOMATA. Mpo?, thigh ; (ttoij^«, 

 mouth. 



I. Sub-order Xiphosura. ^i^og, a sword ; oupa, a 

 tail. The King-crab, Limulus, seen from above, exhibits 

 little more than a huge buckler rounded in front and 

 furnished with a pair of large dull eyes and three ocelli ; 

 a second smaller buckler toothed at the sides ; and a 

 posterior appendage, or telson, shaped like a bayonet. 

 From below, the animal thus esconced appears as if 

 chiefly made up of an assemblage of jointed limbs, of 

 which six pairs encircle the mouth, the basal joints or 

 thighs acting as jaws, whilst the extremities serve for 

 prehension or locomotion. Six other pairs carry gills 

 and are used in swimming. The species are not nume- 

 rous but frequent sandy coasts in many parts of the 

 world, and occasionally leave the water, burying them- 

 selves in the sand to avoid the heat of the sun. The 

 King-crabs were in existence before the Trilobites dis- 

 appeared. The long continuance of so grotesque a form, 

 resembling a magnified animalcule, is worthy of notice. 



II. Sub-order Eurypterida. supvg, broad ; Trrepov, 

 a wing. The species are known as Palaeozoic fossils. 



^ Four species of Limulus, and a small example of 

 Pterygotus, a genus of fossil Crustaceans, some 

 species of which attain a length of six feet. 



