INVERTEBRATE ANIMALS. 65 



Group 162.— Order TRILOBITA. rpslg, three; Ko^o?, a lobe. 

 The species of this order are known only as Palaeozoic 

 fossils. The shell is composed of a cephalic shield, a 

 certain number of free and movable thoracic rings, and a 

 caudal shield, or pygidium. 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 MEROSTOMATA. [j^ripos, thigh; o-To>a, 

 mouth. 



I. Sub-order Xiphosura. ^<'<^oj, a sword ; ovpoc, 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 numer- 

 ous, 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. 



