Small Crustaceans. 103 



All these forms make up a sub-class of Crustacea named 

 Podophthalmia on account of their stalked eyes. 



The sand-hoppers (fig. 55), wood-lice, and fresh- 

 water shrimps, make up a second sub-class, charac- 

 terised by possessing sessile eyes. These also have 

 bodies made up of twenty segments, each of which, 

 except those of the head and thorax, has its own in- 

 dependent chitinous ring, and the two hinder pairs of 

 foot-jaws are used for locomotion. Some of these, 

 like the sand-hoppers and freshwater shrimps, have the 

 three hindmost pairs of abdominal feet arranged so that 

 their joints bend forwards while all the other limbs 

 bend with their joints concave backwards. These are 

 called Amphipoda, to distinguish them from those like 

 the wood-lice, and slaters, whose legs are all directed 

 one way, which are called Jsopoda. 



The king crabs of the Mollucca Islands and of 

 North America form the types of a third sub-class. 

 They resemble the lobster in having the head, thorax, 

 and abdomen covered by a great dorsal buckler, but 

 differ in that there are six walking limbs around the 

 mouth, whose bases are spiny, and compressed, 

 acting as jaws. The eyes are not stalked-, and there 

 is a long bayonet-shaped tail, behind the abdomen, 

 corresponding to the telson of the lobster. The seg- 

 ments of the abdomen are faintly marked in the adult 

 king crab. 



In the past ages of the world, larger allied forms 

 existed abundantly ; other allied fossil forms had three- 

 lobed bodies, and hence are known as Trilobites, and 

 they are only found in palaeozoic rocks. In stagnant 

 pools of fresh water little creatures called water fleas 



